Sometimes, when I have to talk myself into going for a swim, when I'm tired, I'll just tell myself I only have to to do a few laps, I know I'll feel better after I'm done. Then when I find myself in the pool, it's easy to give myself a few bonus laps. Sometimes I lose track of how many laps I've done, after the minimum. So give yourself permission to just do a little bit, to get over the inertia, and then you can reward yourself with more exercise. You know you'll feel better once you are done.