I started doing the same thing a few days ago, in an attempt to get back my habit of waking early (polyphasic experimenting got my sleep schedule out of whack). Something I did differently was, I write in the same box twice - once before I go to bed, something like committing to waking up early, and once after I get up. This solved my problem of getting up, making up some reason to postpone the habit-formation process (or even cancel it to start anew later), and going back to bed. My symbols are a bit more complex, so that I can mark a failure on top of the commitment mark as well, but it would be something like writing the left part of the tick just as I'm going to bed - and no sooner - and completing it when I get up (or crossing it out if I fail).

I think I'll do the same thing for exercising. It's been great so far!