I think the standard advice is to make up your own goals, and let the reward be the fact that you've achieved those goals. (Or add an additional reward to that, like chocolate or something.) If you want to start running, then print out a running calendar and check off every day that you actually do it. Set a goal to run today, or a goal to run three times this week, or twelve times this month—whatever sort of goal seems both significant and achievable.

This is part of how the "ignore everything except the next bit" method is supposed to help. Checking that next item off your to-do list should feel rewarding enough to justify the small effort of carrying out that task.