Hi,
been there, done that. Ok, here's my experience. This is going to take TIME. How long is up to you and your back, but you will progress as your body heals. Push too hard and it's one step forward and TEN steps back. Got it?
This breaks down into quieting, healing, strengthen, adapting.
The first thing is getting symptoms under control to interrupt any inflammation process that might be going on. I'm sure you know about RICE, but have you bought a couple really big cooling pads from a medical supply house yet? I found a daily soak in a hot tub
worked wonders, took me a couple weeks of going every day. But after that, I started to get better. There are other approaches, that's just what worked for me.
Now, let's assume the worst is over. So now we can start therapy (if the doc prescribes it) or light exercise. Here you really need a professional to set you up with a exercise regime that matches your needs.
Next.. you have licked the worst and you feel ready to hit the gym. Set up an agonisingly slow schedule and stick to it. A couple weeks of rowing with one plate (about 10 pounds), then a couple weeks with a second plate. That sort of thing. There are a lot of back exercises. EVENTUALLY you wll be able to do a number of them. But for now stick to a couple your Doc or therapist has said was ok. I'd work your abs on a machine, like a Nautilus. WHen you can do 80 to 100 pounds on the ab machine start doing incline crunches. They exercise the stabiliser muscles like crazy.
I eventually got the the point where I could do Roman Chair exercises, and a rowing exercise designed to stress the lower back. I don't even want to discuss the specifics of those yet.
That is likely 6 months to 2 years away, depending on how long recovery takes.
I like the idea of getting a more forgiving bike. You could also consider a seat with supension. I have been using a Brooks Flyer.
But I would spend a few months working towards recovery before doing even that. You could try spinning classes. You can get noise
deadening earplugs for $1.67 in the Walmart hunting department.
You will know why I mentioned those if your spinning class is anything like mine.
I know the situation looks bleak, but I was worse off. But I brought myself back, and hiked and backpacked and went skiing for years afterwards. You can do it, too.