Try A Couple of Different Things
GTD offers some great guidelines for implementing the approach into your work but the bottom line is, what works for you? What compels you to get it done? If treating it as a meeting with yourself and putting it in your calendar works and you get it done because it's there, then that's the way to do it.
Personally, I didn't start keeping that date with myself until I put it into my calendar. Now, I'm working out consistently 4-5 times a week. That's what works for me so that's what I do despite it not exactly being a "hard date".
I had it as a to-do for a while and then on a checklist for a while and neither worked for me. I would suggest trying a few different ways to see what helps you get it done and then stick with that. Allow yourself to be flexible and find what works for you.
Hope this helps.
"Life is not the harbor. Life is the boat."