My personal experiance:
While walking decided to run a bit on some grass. Ran fast for probably about 5 - 10 seconds. Did a couple more short sprints like that maybe doing 2 to 3 sprints. In a week or so of doing this I felt ready to go and do a actually sprint work out. Walked to a likely bit of hill (up hill is easier on the joints at first) ran up as fast as I could till I needed to stop, walked back down, panting a lot, repeated 6 times if I remember right, went home very out of breath and on a good old excersie high.
I built up the distance and the reps to 8 reps of full out sprinting covering 100 yards or so.
I just went all out for as long as I could, didn't bother with timers or intervals.
You know all those pictures of Adam and Eve where they have belly button? Think about it..................... take as long as you need........................