I use bicycle as my main mode of transport where I live and I always walk and go camping/hiking, and so on. Aerobic exercise is something ive done more easily, also have played football regularly for quite sometime in my life.
The thing is im quite skinny, so in different times of my life I tried to do different exercises focused on muscles. The gym thing didnt last long because the environment sucks big time, and ive found hard to find motivation to do push ups and so on, felt very boring.
Im very thankful to have finally found something Im really excited about, which is brazillian jiu jitsu. I started a couple of months ago and im really motivated for it, Im constantly thinking about it, and its already affecting different areas of my life with it's 'philosophy' (saving energy, being calm under pressure/trouble, using leverage and force of oponent in your favor, etc). It's great because its strategic, intelligent, like human chess, but at the same time its good work out. Also its so much more entertaining and motivating than pure exercise because you are doing with others, which help giving the extra push, as well as help you learn about yourself through constant interaction with other people on the mat.
I really suggest to those that have trouble finding motivation for exercise, to try out martial arts in general (or bjj in particular if theres a school near you). It might be just whats missing in your life