GPS watches during training - pacing guidance

by Chris H
(Kuala Lumpur, Malaysia)

Actual running distance and the marathon :

"Dear Marius,

I've noticed that the "true" distance I run in races is up to 2% further than the declared race distance (eg 42.8 km rather than 42.195 km). This is of course to some degree normal because of water breaks, not running perfect tangents, etc.

I've found this translates into about a 5 second / km pace difference between what I see my GPS watch says I ran during the race and what a calculation of the pace over the race distance (eg 42.195 km) would be. That is, to be sure of hitting the pace you need to achieve a finishing time goal, you'd better build in around 5 seconds per km (more seconds for slower speeds, less for faster ones and people who run better tangents!) into the pace needed to hit the target.

I'd imagine many people following your plan are using such watches.

My question is this. If I'm running outside and relying on a GPS, and am running based on pace rather than either effort or heart rate, would you suggest that I build these 5 seconds into the pace I run? That is, would you suggest I try to run 5 seconds (or so) per km faster than the declared pace, in order to cover this difference? I suppose that indoors one could also use a speed 2% or so faster than the program suggests.

Or would you say that those 5 seconds aren't really important (so ignore them), or are built into the program already (so don't incorporate them), or are overtaken by race conditions so that we shouldn't really worry about them (because they are then essentially built into the program)?

By the way, there is a way, during races, to force a watch onto race distance pace, but it requires the race to have markers every km (or mile) and for the runner to see all of them, and for the runner to hit the lap button (only once!) every marker (not 100% likely!), then use 'average lap pace', rather than just 'average pace' as the current overall pace in the race. Of course, autolap should be turned off.

I had thought a nice invention would be an electronic signal to all GPS watches as they pass through km markers, which could then update race distance pace on the watches themselves. Until then, I'll just use my 5s rule!

With best regards,
Chris H.

Answer: Hi Chris and thanks for your question.

Quite interesting observation you have there, I never thought of it that way :)

However, the 100 day plan is largely based on calculations on the way to hit a given marathon finishing time and it will then naturally have such aspects baked into it. So I would definately not try and run 5 sec faster/km in the workouts for a given finishing time. Especially as the half marathon a few weeks before the marathon is used to determine the actual race pace.

I wish you all the best !

kind regards,
Marius