Either the motivation is there or it isn't. Most of us experience certain benefits that keep us going. After a period it becomes a habit. I enjoy the feeling after a good ride. I enjoy the movement. I especially enjoy the natural beauty that I am taken to with my bike on organized rides and tours outside of the city I live in. Sometimes I don't feel motivated to exercise so I just tell myself to get on the bike and ride for the pure joy of it, no goals. Usually I'll warm up and increase my level of cycling and if not, fine. At least I'm enjoying myself and that's good for me on all levels. Cycling is certainly one of the gateways into the joys of youthful pleasures even though I may not be so youthful chronologically.
Riding with others, friends, local bike clubs, etc is also an excellent way to get motivated. Cycling is a major social avenue for me, meeting new people...etc.