My saying is that you can never have too many bikes or guitars!

I got into biking about 4-5 years ago (I'm 56) because my wife wanted to do triathalons and wanted a riding partner. My only regret is that I didn't get into cycling 20 years ago (I mostly played soccer). Like musical instruments, after a while your preferences may manifest themselves, but when you start out, you don't know what factors are important to you. I bought a Cannondale road bike when I started out. A year later I was taking some mechanics classes and bought a frame and parts to build my own. In addition to just learning some basic bike mechanical skills, I wanted to try a carbon fiber frame (versus aluminum), a compact crank (versus a standard double), Ultegra (versus 105) gruppo, etc. In the grand scheme of things, these are kind of minor differences, but also significant if only psychologically. It's like having an ebony fretboard as opposed to rosewood, or different tuning machines, strings, or bridge on a guitar. But getting a good fit on a bike is like having a guitar with reasonable action as opposed to strings that sit 1/2 inch off the frets -- it will lead to an unenjoyable experience and discourage you. So I would suggest that being comfortable is the most important thing to aim for now.

Having said that, I found that a road bike made me want to ride much more than if I had gotten a mountain bike or a hybrid. I like road bikes - they go a little faster because they're lighter and have less rolling resistance, but there's certainly nothing wrong with mountain bikes or hybrids. Road bikes are targeted for speed more than comfort, but they become comfortable when you get used to them. If you have issues with your back, another option is recumbents. I think you will have to take some test rides, and get some experience before you'll develop a sense of what works for you.

BTW, I'm an archtop player: I have a Moll custom, a '47 Epihone Triumph, a Gibson ES175, a Telecaster, a Steinberger, and a nondescript flattop...