Welcome back from convalescence. The HCRR is a great place to start.
Considering you've never rowed a raft before and, due to your back condition, could suffer significant consequences even with a seemingly minor mishap, you may want to consider starting with a Class I/II flatwater float on some stretch of river that's better known as an open canoe run than rafting run. This would allow you to get the hang of handling the oars, how a raft handles compared to a hardshell, and build confidence along the way before getting onto anything where any real skill is required. Despite what people are about to tell you about the Upper Colorado, with the exception of State Bridge to Catamount and Cottonwood Island to Dotsero, there's at least one nominal Class III rapid on just about every section with easy raft access.
Check out Jerry's River Guide
for information on lots of stretches of river that Buzzards don't even acknowledge.
Good luck starting over,