Thanks for all the thoughts. I'm happy to work on core strength - although I don't think that's a direct cause for my shoulder/upper back soreness.

My LBS switched out the handlebars on my bike. It came with a 42cm handlebar, and he put on a 40cm. As for locking my elbows - I'll keep alert to that possibility, but I don't think I'm doing that.

I sometimes get the same burning muscle sensation when I've been working at the computer for too long. I can see where the prone fly and row would be helpful in strengthening that area - but would the soreness be caused by weakness in that area, or tightness, or both?

Ugh - this whole getting in shape thing has all kinds of pitfalls!