Routine things seem to work well on a checklist. There is a lot of helpful information about dealing with routine things (specifically housework) at flylady.net .
For things like an exercise program, I think you need to schedule some hard landscape to get them done. I walk in the morning before work - getting up before 5 am to get my walk in. While I don't put this on my calendar, I could mark off from 5-6 am for exercise. Before I started walking I kept wanting to do it, but thinking that I didn't have time. Once I sat down and figured out that early morning was the only way I was going to fit it into my schedule at a time when I had energy to do it, it became an appointment
If you think about when you are able to make a place for exercising in your schedule, you could make an appointment with yourself to do it and enter it into your calendar. Until you've figured out when you really can do it and committed yourself to doing it, it's still a someday/maybe.