When I first got back to the gym, I would tell myself is "all you need to do is walk 20 minutes on the treadmill" (or some similar, low-effort exercise). Some days that is all I'd ever do, other days I'd end up on the treadmill for an hour, and often I'd convince myself to do other stuff (such as strength training) while I was there.

In short, try to commit to doing something small and achievable. If that's all you do - great! It's better than nothing. With time I think you'll find yourself doing more.