ChatGPT was my AI marathon coach that helped as a pacing strategist for my training plan for Perth running festival marathon. See conversations below and what was my biggest mistake on race day.
I had ear marked the Perth running festival race for 2025 after doing the half last year. Running through the tunnel and having nutrition and pacers was key for this decision. It sold out quickly last year so I bought my ticket early and lucky cause it sold out again this year.
For me the Perth running festival marathon was the end of a series of races (Perth Half, Perth 32, City to Surf and Sunset Coast 34km) that I did to get myself back training and racing.
Having a race planner helped me a lot for this training as I had a lot going on so was good to remind myself of A vs B vs C races.
Table of Contents
I was pretty confident going into the running festival as felt I had done enough to hit a pretty big PB from 3.59. I have improved a lot over the year since the last attempt.
I have never went balls out on a race where I was pushing to the limit, I’m generally a conservative racer focusing on maintain pace and living to fight another day while slowly take small chunks of time off my PBs.
Felt 3.45 pacer was “easily” achievable but 3.30 is audacious so on 12th of October 2025 I said fuck it and wanted to leave everything out there and set that as my goal.
My logic was simple if I can do a half in 1.45 with limited run training I should be able to maintain the pace for a full marathon with a 10 week program. Garmin actually predicted 3.22. I did know its not as simple as that but wanted to challenge myself.
Firstly my series of races and training plan, my ChatGTP race pace chats and then some lessons learned.
Negative spilt Perth Half Marathon in 1.45
An absolute horrible day.
Thunderstorms ked to race being delayed by 1 hour so I got there late in case they cancelled it. The race had literally started when I got to starting line so had drop my bag and race from the end.
With the wind it led to a controlled first 10km and then on the way back just let rip and was gun hoe to get my 1.45.
Really happy with this negative spilt. It is clear to see from 11km onwards was hitting my sub 5 km/min targets and even finishing faster pushing sub 4.50s.
Looking at training activity a lot of my training was cycling more than running funny enough.
Stepped Perth 32km finishing at RP in 2.56
Another miserable day. Got a 30K PB in (2:47:44).
The course is from Marathon club down towards Maylands golf course and back. All on walking/cycling track. Well supported with aid stations and competitive runners out there.
This was when I started using ChatGTP to get some general advice on how to approach the race,
For me it was a controlled z2 run even though in rain. It got tough to increase the pace but was sub 5.10 for the last 6kms.
City to Surf 12km in 59mins
Huge crowds at this so was trying to avoid people as much as anything for the first 2kms up past kings park. Reflecting setup a nice negative spilt again.
What ChatGTP recommended and I followed for race day, The two kms over 5min/kms at 10/11 was running up hill. I didn’t run an extra 9km cause I was shattered after tbh.
Got a couple nice PBs at this race. Was wrecked after though and took a good week to regroup. I do find going faster a lot harder on the body in terms of recovery. I’m just more slow and steady.
My slow is just getting slightly quicker.
Sunset Coast Run Not a Marathon 34km in 3.04
It was my last big long run of my program. Had huge stomach issues. Had dates just before starting and tried new carb drink. One or both didn’t agree with me. Delayed stopping till toilets which were then closed so had shuffle to the next which was tough.
It was a 4 lap race and you are out there with people doing a marathon which was 5 laps and you had a half and I think a 5km fun run with kids there too so the course was busy to keep you distracted.
What ChatGTP recommended
Was actually perfect as a last long run while simulating race day. It was a hilly sand dune course so some accelerations but in general kept steady.
Looking back now the rain wasn’t too bad compared to the heat.
This race did make my shin splints worst and wanted to rest while hoping to not lose fitness before the big race. Then moved house and was balancing life while resting.
Perth Running Festival Marathon in 3.54
The big day.
Had get train from Glendalough at 5am as couldn’t drive and they want you there early so no other train was leaving Yanchep station till after 6pm so had no choice. Train was full with runners which was a cool vibe all by itself.
Nutrition I felt was on point. Crumpets 3.30am. Powerade on train. Had gels and salt tablets. Used race belt not nerds belt cause they had nutrition on course and recently felt its a bit awkward. No stomach issues like Sunset race..
Did pre-race warm up of 1km and lots of stretching. Put on warm up cream on shins and quads for good measure prior as had some shin splints on build up.
One thing I didn’t take into account was the heat. I thought starting at 6am I thought it should be no issue so wore no sun cream!!!
Placed myself with the 3.30 pacer compared to last year doing the half and City to Surf couldn’t get into good position due to crowds.
First 5km my legs / quads kind a tweaked but put it down to just warm up niggles and would go away. It did and was something annoying me a few km later.
Looking now my first 5km was far too quick. This was trying to stay with the pacer. At the time I didn’t realize this. Pacing at the time I felt like I was holding okay. Thought I was just slightly over 5.00min/km pace.
Didn’t feel under pressure. Was steady on inclines up towards casino and solid going through tunnel. Thought the garmin might of been glitchy through the tunnel that might of been a sign that I was starting to struggle to hold.
Coming out of tunnel as the 3.30 pacer was running towards me and felt I took pedal off the gas so picked it up a little and skipped the water station.
Half way looked at the watch and was 1.46 and I thought okay the plan is still possible.
.
The next section from 18km to 29km marks. was freeway south towards applecross and back. It felt like running in the Sahara. Sun was beaming down with no cover. It felt pretty hot.
Thats where it all fell apart.
Had a gel and got stitch in side so took my crampfix. Ironically just 2km later my quads stared cramping on the 27km mark.
Water stations from here on were a bit of a mess with volunteers struggling to supply the dehydrated runners, I just started getting slower and slower. Cramping got worst and worst.
Tried holding 5.40s then stay under 6 and then just stay running.
Watching the 3.45 pacing group glide by just out my the UWA boathouse was demoralizing but not even being able to try keep up was a reality check.
Now the goal was to just finish.
Thought about quitting but just kept plodding along. Trying to shuffle and keep moving. I have never DNF and had no intention of starting.
Towards the end there was several people getting medical attention or just sitting down from the heat so I think at least having salt tablets helped avoid that situation.
Last 500m over the bridge and down into Optus I just tried look like a natural runner and hide any signs of pain. Extra annoying they seem to add an extra 500m onto the race for some reason. Last thing I needed.
Finished officially in 3.55. It was a tough day out.
Lessons learned
Own pacing vs race pacer
I tried sticking to the 3.30 pacer with plan to keep them in sight. That’s a 5.00min/km pace when in reality my race pace is probably closer to 5.10min/km. The 3.45 pacer was at 5.19min/km (see marathon chart here) That would of got me done at 3.40 which would of been very respectable.
Weather Awareness
As a triathlete I’m always looking at the weather and I can’t believe I ignored it. When you have people lacing on sun cream maybe you should consider it too. It was 19°C starting by 10km 26°C and finished at 31°C. Lucky I wore a hat.
Never Skip Leg Day
I’m slightly time poor so was running at lunch instead of going to the gym and doing leg work. That cramping hasn’t happened in a long time so I think the strength work really helped. My strength work is based around Single Leg wall squat, Bulgarian with heel raise and Split squats. Try to do hip activations and stretch prior.
Expect Worst Case
I didn’t have any deep heat or anything I could put on my quads to quell the insufferable cramping. It helps even the small bit to help getting it done. The one crampfix was used for my stitch. Usually on Ironmans I have a hail mary kit of deep heat, crampfix and pickle juice. That would of helped.
Negative splits pacing
Negative splits were what built up my confidence and it was a mistake to not replicate that again. Instead going out too fast which definitely did not help my final outcome. Also, I would like to finish strong not start strong.
Total time awareness
Even though the race went to shit I was still looking at my total time towards the end so knew where I was at and still could get a PB even though not what I wanted it still was sub 4. Plans change I had adapt accordingly.. For my last Ironman I didn’t know so when finishing I was looking at the clock instead of enjoying it.
ChatGTP as a Pacer Strategist
I tried give as much context as possible to get good recommendations. I really liked being given multiple pacing options and nutritional advice. It knowing my goals and training progress was great. I think even if I gave it more context such as my strava workouts it would of been even better.
It will cater towards your goals so giving it feedback on how you felt and actual data is essential for good outputs to get a realistic racing pace plan.
This is what ChatGTP recommended for my marathon. I wonder if those first 5kms would of made all the difference. At same time my legs might of still cramped due to heat and lack of strength training.
🧭 Chatgtp Recommended Marathon Race Plan
| Segment | Target pace | Focus | Notes |
|---|---|---|---|
| 0 – 5 km | 5:10 – 5:15/km | Settle, breathe, stay relaxed | Hold back a little; heart rate under control, find rhythm. |
| 5 – 20 km | 5:00 – 5:05/km | Lock into “comfortably hard” | Feels smooth; you should be able to chat in short sentences. |
| 20 – 32 km | 4:58 – 5:02/km | Stay focused on cadence, nutrition, and form | This is where your endurance training pays off. |
| 32 – 42.2 km | As steady as possible (≈ 5:05/km) | Dig in but don’t sprint early | Mentally divide into 2 × 5 km chunks; aim to finish within 3 h 30 m – 3 h 33 m. |
Projected finish: ≈ 3 h 31 m – 3 h 34 m
Most annoying thing there are no more marathons in Perth this year to redeem myself and at least do one with a 3.45 pacer to prove my pacing calculations are right..
You can see the full chat here with ChatGTP getting run pacing and nutritional advice.
Knowing what I know now from Agentic Engineering course I would setup my Agent a bit different:
- Refined system prompt vs just user prompts
- Convert strava files to markdown as inputs
- Capture more detailed race reports after each session to couple with the data.
If this is something you would like to try out if I build it just email shane @ shanedrumm.com