Thanks guys for all the good thoughts. I will see a doctor if the problem isn't resolved by clipless pedals and by riding more often. I do try to keep the cadence high and not mash.

The annoying thing about this is that it has come on very suddenly, with no change at all that I can think of. My bike was fitted when I got it in July and I have been riding with no discomfort for rides of 20 to 55 miles (except for hand numbness which is a different thread).

I have also realized that I ALWAYS put my left foot on the ground first when I stop, and I am going to try to alternate that. So much to think about