This article claims a linear relationship: Shed 1% of your weight in fat and you will run 1% faster. In Paco's example, going from 81 to 75 kg is a 7.4% loss and has the potential to reduce his 10k time from 53 min to 49 min.
The Impact of Weight Loss on Marathon Running | Chron.com
I am buying this. I see a huge difference in my ability to run related to my body weight. Of course there is a limit, as the article states, when you get to low body fat percentages.
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