Sign up for a charity run/walk/bike ride. Make sure that the event is a couple of months in the future, and that what you need to accomplish (eg. run 5K, walk 10K, bike 10K) is beyond what you can do now. That way, you need to work at it consistently (C25K program, for example) for the period of time in order to achieve the goal - and since you are raising money on the premise of doing this, it's hard to back out.
Once the event is done, you have established a healthy habit, and raised money for a good cause.