Upper leg pain

whizzlers

Hi, I have been able to comfortably run 5k for about 5 years now, but I decided and I wanted to push myself a little further and enter a 10k, however every so often I have a pain that run from the top of my leg when I run and then continues for a couple of days. I walk to cool down and do plenty of stretches after and I do a warm up before I starts, but it still always comes back. Is it due to the fact I have been doing longer distances that my body is used to? any help would be appreciated as I am now covering over 8k, which is a massive achievement for me.
Thank you xx