Based on advice I've gotten here, I've converted an old MTB frame to give me a much more upright posture. I've added a stem extender, and swapped out the straight bars with trekking bars, and it's made a world of difference - it's night and day.
A remaining issue is that the more upright posture is putting a lot more weight on the saddle and the parts of me that come in contact with it. The seat is already a little wider and more padded than a "serious" cycling saddle - I did that a while ago so that I could ride without wearing special clothing. I've noticed that if I make a conscious effort to push myself back on the seat, it seems to support my sit bones pretty well. But what's happening is that, as I ride, I tend to creep forward until I'm kind of perched on the nose of the seat, and that causes definite problems. (And yes, if anyone's wondering, it IS still an issue at 59! )
The seat is as far forward as it goes on the rails right now, so I can't move it up. I saw the forward offset seatposts, but they're mega-expensive, and I don't want to spring for over $100 to try a new post. I was wondering -- is it possible to just turn the one I have around? That would give me an extra inch or so to play with. Or is there some problem with that?