Good news, after my knee-induced hiatus over the summer I've tried some light spinning units, and my right knee appears to be okay on the machine. Hopefully, come reasonable weather and daylight, this will translate into pain-free cycling and some more miles. At worst I'll seriously have to rethink my setup, since apparently there is a way for me to put down reasonable power (considering the horrible shape I am in) without having to suffer three days of pain.