Find a uncompetitive run in your local area thats bout 4 months away, pay your entrance fee and follow a training program for it. 5 - 10 kms is a good distance. If the run is a big one, training programs will be online, failing that use google or buy a book on the subject.
Bear in mind that when you first start training everything hurts. This feeling soon fades and you should see the result of your productivity.
I'm only using a run as an example, what ever you do keep setting targets to meet. This will get you over any emotional peaks and troughs.