Need some advice on body building?

I used to sprint,and did it very well.I was a happy sprint champion and ended up having a sleek six pack and well toned body when i was 17,now I am 24 and would like to put on a little more muscle mass.I would want to increase the size of my biceps and also the individual sizes of the Six pack blocks.A potential problem I see is that, my muscles are already stiff as rock all over(Bruce lee types).I dont look skinny,neither do I look built.I would want to put more mass into an already stiff abdomen and arms.Any advice?