If you were to train more (say running) and lost 10 lbs through that, and say you trimmed 45 seconds per mile, there is no way to determine (outside of a controlled experiment) if the speed gain was from the 10 lbs lost, or the increase in running/training volume that lead to the weight loss. Maybe you just got faster because you ran a hell of a lot more. Or maybe it was from the decreased weight, or likely from both.

Anyway, I think it's pretty obvious someone not carrying extra weight will be faster (assuming they are fit). And I have also heard 2-5 seconds per mile per lb lost.

Good luck, I'm sure whatever the speed gain, you'll be faster with a little weight trimmed off. Report back after you've done it.

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Empire Tri Coach
Team Gatorade Endurance
USATF Coach | NYRR Distance Pacer
Dad of twins