In reply to The New NickB:
Did you have your shoes laced any differently, or might the forefoot have been tighter in any way than in your other runs? It sounds a bit like a condition I've had a few times, although physios and podiatrists don't seem to be able to decide exactly what the problem is in my case.
I suffered hugely sore feet in a half marathon three years ago, and the general thinking was that the forefoot was too cramped so I was developing metatarsalgia - a nerve running between two of the metatarsals was being squeezed together, and was very sore. It felt like it was burning, and didn't go away when I took my shoes off. I subsequently got orthotics, which seemed to sort that problem but caused further problems with my calf, so I'm now running without the orthotics. I haven't had as much pain since, but I did do a half marathon yesterday, which was the first time I'd run much distance on road for quite some time, and there was some pain at about the 8 mile mark, which hung about till the end of the race. It's still a bit tender.
A related condition is Morton's neuroma. This is what my physio think I've got - it's a kind of lump on a metatarsal nerve, which gets inflamed when under pressure.
Most of my running now is off-road, so I don't think I'm quite so prone to the same problems any more.