Wellll....first of all, I never walk. Period. As far as sprinting goes, I wouldn't do anything less than 200 meters. Preferably a 200 meter recovery jog, but you can stretch it out as far as you like. 400 meter sprints are a lot better than 200 meter sprints. And 600 meter sprints SUCK! (that means that they're really good
) As I get back into shape, I'd like to do 600 meter sprints with 400 meters for recovery. About 5-6 of those is plenty. Generally 400 meter sprints are recommended. The 600 meters is personal preference...because I've done 200m sprints, 200m, 300m, 400m, 500m, 600m, and 800m. I just hated the 500m & 600m the most because they were the toughest.
I did 200 meter sprints with a 200 meter recovery the other day, but that was because half of the track was covered in ice. You can jog as slow as you need to, but I'm very, very much against walking. If you get in the habit of jogging, as you get back into shape your pace will pick up on your recovery laps.
And record your times! I know you're probably not going to be running in track meets, but that is the best way to gauge your progress.