I'm 49 next month and had an L4/5 fusion 2 years ago. At the time I'd been playing around on a mountain bike but was keen to try a roadie. Went to the LBS and told them I wanted something light (easy to lift, not weight weenie OCP) and comfortable. They recommended a flat bar road bike but I was pretty keen on drops so ended up with a 2005 Giant OCR1 (105 groupset) and they flipped the stem so I was a bit more upright.
Early on it was a bit sore on the back but I think it was just the body getting used to both the new ride and the new back hardware. Over time it's gotten to the extent where it doesn't bother me much at all - other than a mild stiffness after a couple of hours. I just sit up and stretch out whilst riding - and I've seen plenty of people younger than me doing that
The OCR's are considered 'relaxed' I believe so I don't know how I would have gone with one of their other frames (I don't know enough about geometry differences to comment).
My surgeon was happy for me to be riding again after a few months. He very much pushed me to continue swimming which I'd been doing for a year (I'd had a laminectomy 12 months before the fusion). I fear a bad fall but I feel strong and my back is solid so I 'just do it' and be as careful as possible. FWIW, I'm doing my first triathlon next weekend - figured I was already swimming and cycling so thought 'how hard can it be?'. The run training has been way harder than any riding.
Having said all that, a neck fusion may be a different kettle of fish, but hopefully once it's fused your doc will give you the OK.
//k