1) depends on your riding position. Upright means you are using mostly legs. The more bent over you are the more the butt and back come into play.

2) Do one long ride a week. Most weeks try to make it a little longer, usually about 10%. In between do as many rides as you feel like, but don't walk around hurting a lot. One thought is per week... a long ride, a hilly ride, a short to medium fast ride,
and any short rides you feel like doing.

3) Most skinny guys are skinny for a reason. You can try bodybuilding. And it will help, but your fate is to be skinny. Count your blessings.

4) Cyclists should do a little exercise that is weight bearing. Jogging, hiking, weight lifting. Stimulates bone growth.