ok.. here's my little bit that i've recently learned...
as you are training.. start with about a 6min run.. 1 min walk (lowers heart rate), then 6+1 etc.. till you've run your distance.. gradually increase that to a 10+1 (run 10, walk 1) dosn't really affect your timing and gives you that little break during... (so i've been told.. I'm doing a 7+1 at the moment.. ) when you do the actually race.. if you're not going for time but for completion, (ie racing for the top spots), then run the 10+1 and apparently it won't affect your overall time that much.. ps.. drink lots during the walk..

so.. i could be doing my first half marathon in october.. only for completion... lol

good luck..