Oops ...first little setback

I went out this morning to a running track to do some 400 metre sprints. I aimed to do about 12 over a period of maybe 45 minutes. In the past few years the best time I've managed is 1 min 14 secs and I was hoping to get near that with at least one of my laps. I started off with a good warm up consisting of some slow laps and an easy hill climb followed by 10 minutes of stretches. I started off the serious laps with a 1 min 18 secs and followed that up with a few in the mid 1:20's but on lap 9 I felt a little twinge in my ham string and stopped right away and walked home. In the past I'd have pushed on and finished my training but hard experience has told me that that just leads to more problems.
My ham string has been a bit tender all day but nothing that is causing me too much concern. A good nights rest followed by an easy run tomorrow should see me okay again, I hope!