This thread is great, I was about to ask the same question. Trying to get the cadence up already, but I'm doing the climbs so slow I go down to 70 rpm. Should I try to stick with the flats until I can climb faster than 8 mph? And I do pronate, wondering if that's why my foot seems to be finding the limits of float on my SPDs. I realigned them, but if it stays a problem, I'll have to go back to Look.

As for the OP, maybe you're just getting back into it too quickly? I haven't gone more than 20 miles yet, even though my cardio feels like it could do another 20. My thighs tell me otherwise over the next day.