As suspected, I have shin splints.
Good news is that they have not been caused by over training necessarily, but by a combination of pronation, bunions and hypermobility in my foot and toe joints.
The podiatrist was amazed by just how flexible my foot joints are (something I'd never really thought about, assuming that everyone's feet were the same), but the problem is compounded by a very stiff big toe joint (caused by bunions) and then even more so by promoting inwards...all putting extra strain on the muscles of my inner leg.
So, I'm back next week to be fitted for orthotics, which I will then have to gradually wear in in normal shoes for a couple of weeks before trying them with running shoes.
While I'm relieved to finally find out what the cause of my running problems is and know that it's something that can be sorted to allow me to keep running further in the future, I'm gutted to have to pull out of the Swansea 10k after all the work I've put on - it would have been my first 10k race and the same thing happened to me 4 years ago when I got ill with Crohn's