Apparently, I'm doing it wrong
I took my bike back to the bike shop to see about getting the gears working. The guy there said there was nothing wrong, but that you simply can't count on using "cross-over" gears, especially in a bike as old as mine. That is, I had been trying to use the large (outer) gear on the pedal crank with the largest (innermost) gear on the rear wheel when going up hill. Apparently, this runs the chain at too sharp an angle and makes it susceptible to coming loose. So... lesson learned. You can't use all 12-speeds on a 12-speed bike, apparently. I did not know that.
I purchased a pair of cycling gloves to keep my hands from getting numb, but the bike shop guy dissuaded me from buying toe clips. His feeling was that they don't do much except keep your feet on the pedals, which has not been my problem. To increase power, he recommended a cycling shoe and special pedals that snap together. I've seen those, but it's too much of an investment right now.
Meanwhile, the tendons above my knee are very sore. I presume this is from unaccustomed use of those particular muscles, and not a tear or strain - I hope. If so, it should feel better in a few days and I can go ride some more.
I purchased a pair of cycling gloves to keep my hands from getting numb, but the bike shop guy dissuaded me from buying toe clips. His feeling was that they don't do much except keep your feet on the pedals, which has not been my problem. To increase power, he recommended a cycling shoe and special pedals that snap together. I've seen those, but it's too much of an investment right now.
Meanwhile, the tendons above my knee are very sore. I presume this is from unaccustomed use of those particular muscles, and not a tear or strain - I hope. If so, it should feel better in a few days and I can go ride some more.