Getting back to walking

I've decided to wean myself back onto walking. It's nearly a year since my back went, and none of the exercises seems to be fixing it fully. I therefore intend to try putting my rucksack back on and heading out on a few trial walks. Perhaps the cause will also be the cure.

My first plan was to walk the guided busway between St Ives and Cambridge, but I have walked that many times before, and the stretch near St Ives is flooded (as it always seems to be).

I'm therefore looking for another route. Ideally it would have the following criteria:

1. Between 10 and 15 miles;
2. Preferable on a good surface due to the weather;
3. Has public transport at regular intervals in case my back goes
4. Optionally has lots of interest to see and do.
5. Preferably within easy access of Cambridge.

I's prefer an easy (tarmac / gravel) walk as any slipping is likely to aggravate my back. So I'm thinking old railway lines, or possible canals through / between towns of cities.

Any ideas?