Thanks for the suggestions! I did consider the various questions about where I would be riding, duration of ride, etc. But I didn't have much to compare with besides the discomfort of the Trek and the effort required to pedal the Murray. I even wondered if I've been developing balance issues, but I've done fine with two rentals on vacations in the past few years. Both were hybrids, one resembling a mountain bike and the other more like an 18-speed cruiser. I think I'll go with the idea that the bike is a little large and not the best suited for my intended use, and start test riding other bikes.
My budget is extremely limited right now, so I'm checking out some used bikes. I'll be looking at a 16" Giant Boulder womens mountain bike before work today. I don't expect it to be the right bike for me, but it'll give me the chance to see how it compares with the misery of the Trek vs the fear factor of the Cypress.