Very nice work! I like the anatomy, and have just a few crits
1- The pose = He needs a bit more forward balance IMO, so it looks like he is about to take off sprinting, or ready to run at that moment. A little hunch forward and the hips slightly more forward, you should be there.
2- The body proportions = I agree he should be athletic, even muscular, but for a runner he is too thick. I think you should go down to level 1 and use the move brush to adjust proportions. Slim him out just a bit, he can be ripped, but this guy is body builder big.
Would you mind posting the base mesh?
other than that its awesome work. Looking forward to seeing it develop. 