Originally Posted by
noimagination
You sound pretty experienced, so it's unlikely that I know better than you. However, when I notice I'm having a similar experience (catch people on hills, they catch me on flats) it is usually because I'm working a lot harder on the climbs (because climbs are supposed to be hard) and not as hard on the flats. So, someone maintaining a more steady-state effort might get passed by me on a climb (when I'm working harder than they are) but might pass me on the flat (when the ratio between my power output and theirs is different).
Steady state power is your best strategy to delay muscle fatigue for as long as possible when doing very long rides with or without lots of climbing.