How Much Can Losing Weight Increase Pace?

This weekend I ran a 1:28:19 half marathon. I started training 7 weeks ago after not running frequently for about 1.5 years.

I finished in 23rd place, but 1st place in the runners with love handles and man boobs category. I’m 5’11” and 78kg and while I’m not fat, I’m about 4kg heavier than when I was at my fittest (cycling a lot, playing rugby at a decent level).

I’m planning to run another half marathon in July and was wondering, if I changed nothing else with my training, and just dropped those 4kg, how much of a difference could that realistically make to my time?