Serena - I found this happened to me, too... it still does if I don't follow my own advice: STRETCH! Honestly, I could not get my head around the idea that I had to stretch BEFORE I ran... I mean, a five-minute, brisk walk should be enough, and walking after, and stretching after, right? Well, when I do that, my calves and shins are sore the next time I run. Of course, I have no idea if it is just me, but if I give myself a goos solid four or five minutes stretching BEFORE the warm-up walk (and nowadays ten minutes stretching AFTER!) I have a lot less trouble. Worth a try?
Good luck! Heather
EDIT: Actually - even today, I found I had to go the first mile before my legs 'caught up' with my brain!!