First off, for most of the people you hear commentating on most televised sports, that is not their sole occupation.
Journalism seems to be the de facto route (Allen, Buxton etc)
Ben Edwards was a mechanic/part time racer. I don't know how he got his start in broadcasting but he also has his own business:
http://www.entreprix.co.uk/
Good luck. I have tried commentating on races (in private) and i run out of things to say pretty quickly.