back on the horse

I got back on the horse wednesday after getting thrown off at mile 18 of my planned 22 mile run on Monday. I went for 10, no problems really, just that nagging demon saying a little louder that it's always easier to walk. The old football coach is right, once you quit something, it makes it that much easier to quit the next time. Why is that? Why is it harder to go for a run when you've taken a few days off? Why is it harder to resist that cheese burger when you just gave in and had one last week?