Effect of Weight on 5k Running Pace - Does it Matter if it is Muscle or Fat?

Effect of Weight on 5k Running Pace - Does it Matter if it is Muscle or Fat? About two years ago, when I achieved my PR in the 5km, I found that my weight loss was resulting in as much muscle loss as fat loss, which is not good for someone in their mid-50s. So I decided to stop trying to loose weight and concentrate on building some muscle, and I have put on about 5kg - I suspect about an even split between muscle and fat. That is the context. I tried some on-line calculators that estimate the effect of weight gain on running pace, e.g. https://runbundle.com/tools/weight-vs-pace-calculator Oddly enough, my weight gain from 81kg to 85kg suggests my PR of 22:22 maps on to 23:17, which is pretty close to my most recent maximum-effort 5k time of 23:26. So it is reasonable that the heavier you are, the slower you will be, but does it make a significant difference whether it is muscle or fat (few fast distance runners are very muscly or very fat) and if so, how much difference does it make? I think my muscle gain is mostly core and upper body. This is probably not going to change my training too much, I am mostly just interested in the basis for the calculation.