right... reread your post...
I'd hope you've got good shoes, you just done a half - well done!
def see a physio. if it is itbs it can take a while to settle down.
if your overpronation is bad, they'll probably suggest a trip to a podiatrist for orthotics (you always get orthotics if yuo go to a pod

I don't like this route anymore, but again I am opinionated about some things so aren't always impartial!!!
another course of action for the mean time is plenty og stretchingg.
get nice and warm, stretch hamstrings, ITB, piriformis, quads and calves - need to keep all these areas supple and relaxed google for stretches, look for the 'figure 4' itb stretch the leaning one doesn't really work imo.
also look for myofascial release, itb doesn't stretch (it's tfl that your stetching above, but people use terms interchangeably) so you need to massage the band to try and release it off a bit.
if I get change later I might flesh this out, but i'm running out of time at mo!