I went to the sports clinic with what I thought was a hip problem but it transpires that I had tight quad muscles - funnily enough they don't seem too bad this week. On reflection I'm not sure if being flat footed is the cause of tightness further up the legs, it's just that they noticed I had flat feet.
Are the 2 related ? Don't get me wrong I only start to feel a tightness after about 2.5 hrs of running so that may be down to not being as fit as I once was - perhaps I should quit these long races now.