If you are unfamiliar with meditation see my earlier post entitled "How To Meditate."
Here is my instructions for meditation while running:
(1) Choose a distance to run.
(2) Start running and do not stop (unless you become injured) until you complete the distance.
(3) When your mind starts:
(a) making excuses for why you should stop or cut short your run, or
(b) judging you, or
(c) comparing yourself to others, or
(d) thinking about how much distance is left, or
(e) starts making plans about how much you will run in the future...
Say your meaningful word internally until that mental chatter stops. Usually one time is enough.