I was roughly the same age as IAMHORNYDAN's husband when I started to run. My now-ex-wife was not clinically depressed at the time but she was certainly overweight - and so was I. Once I started running, though, I lost a lot of weight.
Physical reinvention is a very common theme among new, middle-aged, long distance runners. So are marital breakups. I think that when one partner undergoes a cosmetic transformation it can be really threatening to the other. I suspect that IAMHORNYDAN is feeling left behind because of his change and also because marathon training can be hugely time consuming. He's probably now spending a huge amount of his free time training in the company of a new group of friends.
I'm just not into him. Especially now I'm fat I hate feeling like I could snap him in half.
I don't buy it. Yes, he weighs less and his old clothes are probably baggy on him but he's probably super toned and has tons of endurance, to boot. It sounds preemptive, as if she doubts her ability to keep him interested and she'd rather cut him loose than have him dump her.