How to Build a Real Marathon Race Plan From Your Garmin Data

Garmin's race predictor gives you one number, for a flat road. Race day is a course: bridges, hills, a crowd that pulls you out too fast, and whatever your training actually looked like.

So for NYC tomorrow, I had Claude Code build my plan from my real Garmin data instead.

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A guess vs a plan

It reads every run from your last 16 weeks, finds your course and every climb on it, pulls the race-day forecast and asks what went sideways in training. You get a pace for every mile, not one number.

What it told me

My goal was 2:59. My data says 3:36.

My 3200m test and my 10/18 run say 3:19 on a flat road. But I haven't run over 7.3 miles in six weeks, so it added 14 minutes for durability, plus 3 for the NYC bridges. The forecast is close to perfect, so weather adds nothing.

It didn't round toward what I wanted to hear. That's the point.

Every hill gets its own pace

Rules for when it goes sideways

Heart rate cap of 158 through mile 20. At 20, if my heart rate is under it and my legs are fine, I drop 10 seconds a mile and race the last 10K. I always hold back, so that rule is there to push me.

How to build yours

1. Get your Garmin Personal Key and save it where Claude Code can read it.

2. Paste one prompt. It pulls your training, finds your course and the forecast, interviews you, then shows its math.

3. Race the rules, not the number.

I wrote up the whole guide and the prompt.

Get the guide and the prompt

NYC is tomorrow. I'll report back with my real splits against the plan.