I've been training for my 1st marathon for the last several months & have lost a lot of muscle! I usually run 8-10 miles 4 times a week and then one day a week will go 15-20 miles plus I'm lifting 3-4 times a week. I think it's all about eating enough. I know that I have not been eating enough calories. After losing a lot of weight I have a fear of getting fat again, so I constantly under-eat!