My PRs were set when I was 17 , at 5'10" and weighing 150-152lbs. My estimate is that I was about 15% BF as I had no definition, was "skinny" from running, but could not see my abs or anything that looked like muscle.
5k - 19:30 2 mile - 12:06 1 mile - 5:28 800 - 2:22 (Ran 25-30 miles/week)
I'm 28 now, still 5'10", but weigh 190lbs. I have lifted significantly for the last 5 years and would say that I have about 15lbs more muscle now than I do when I was 17, but also more fat at this point.
If I were to get down to 165lbs and have 12-13% BF (I have gotten there before in the last 5 years), and kept training the same (let's assume I've been running all this time and keep running "fitness" the same), would I be faster at all these distances? or slower?
Generally, I know there is a 2-3 second/mile benefit to losing fat/unneeded weight. I'm curious though, if I'm 10-15lbs heavier than when I set my PRs but leaner and have more muscle, how that would translate to running? Is there any reliable science or anecdotes that could indicate what to expect? For what it is worth, I felt like I really lacked any real upper body strength when I was 17 as I could have maybe done 10 push-ups at once, couldn't do a pull-up, etc... which now I can do those things fairly easily.