In reply to phil27uk: yeah be careful, you could be heading towards achillis tendonitis. If its not your gait, then how old are you. I'm early 40s and found going from zero running to 10K in 6 months caused achillis problems, especially on roads.
Lots of stretching before and after and keep a slow pace early on may help as well as offroad,
Don't want to worry you but it could lead to major problems if you ignore it.