A common mistake is to do massive amounts of press-ups or situps to try and build muscle. If you want to build muscle mass you should do low repetitions of high weights. This is a little trickier for sit-up type exercises without access to a gym, but try doing it with a weight on your upper chest area. For home exercises though, a set of adjustable dumbells is very useful. Personally, I play hockey for about an hour or two normally once or twice a week this time of the year, normally in matches or training. Rugby before Christmas, and tennis in the summer. I also go to the gym three/four times a week to work with weights and go swimming. In the summer I also cycle a lot, and run in the winter, although less running this winter. I got told, that way is unhealthy for people under21 becuase our body is still developing, so the best way for that age is high reps low weights, it doesnt cause as much damage