I think one thing that separates me from my randonneurring peers is that I don't 'get' climbing. I mean, I like the reward of a gorgeous view at the top of a peak, but I always wish I can just fast forward through the part that involves an hour of grinding away at 7mph. It's not a threshold thing, as I'm willing and able to sustain hours of discomfort for a specific goal, but it's not something in which I revel.
While I also don't mind pain (and am fond of the saying "pain is weakness leaving the body") I don't find it to be a requisite component of enjoying the ride. I suspect that might be why I don't enjoy climbing. I enjoy long rides partially for the challenges that they impose and pain is a signal that the challenge has been worthy; but I don't seek out pain for its own sake.
Have you looked at getting orthotics or having someone else look at your pedal stroke?