Good luck with your recovery, Martin!
I took quite a long time to get back on my bike after my illness - 8 months - but I got a lot of my endurance back through walking. I went from having to rest after walking down one flight of stairs, to power-walking up a steep hill and back down again (a 4 mile round trip).
I listened to what my body was telling me, trying harder when I felt able to, and resting as required. I upped the distance, frequency and speed of the walks in a steady progression. When I finally got back on my bike, I was able to do (very slow!) hilly 20 mile rides almost immediately.
I don't have any experience of neck rehab, but my approach certainly worked well for my legs and cardio-vascular system.