There are some days where I don't feel enthusiastic about going for a ride but I feel I "should" either to keep up with my conditioning or if, for some reason, the next few days look like I may not get out (weather, schedule, etc). What I'll sometimes do is just go for a ride, mentally giving myself permission to cut the ride short if I'm not enjoying it. Invariably, once I get out, the endorphins, or whatever, get flowing, and I end up doing the full ride I targeted.