Hobby joggist here who was not underweight nor am anywhere near overweight but started cross training to supplement running. I'm at about 7 hours per week of running and plan to swim 3 hours per week. But, was it worth gaining the extra muscle and fat to gain more non impact strength/endurance work? Where does the extra body weight start to reduce the effects of "fitness"? This is rather complicated so I didn't I'll get many serious answers. Let's say weight went up 10%, half muscle, half fat.