Join a club that focuses on exercise (running club, cycling club, etc...), you will then justify it because you:
1. Spent money on it
2. Can't be lazy because someone is telling you what to do
I'm in a swim club that practices 4 days a week. I don't always make all 4 but I consistently go to 3 of the 4 per week.
---
http://i.imgur.com/2m6g0Yu.jpg