Our expert says:
Best bet, as always is to actually see someone about, because they're always going to be better able to make a diagnosis that I can over the net.
There's a condition called Morton's Neuroma, which springs to mind because of the location (I think) and the fact that it is worsened during walking. But that's not a diagnosis and I may well have misread or misunderstood something in the description, so I really would suggest that you see a podiatrist about this. I suspect that it is nerve related, perhaps brought about by increasing either speed or distance recently, or perhaps shoes?
But a diagnosis is needed and so while I hate passing the buck, that would be my advice!
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