I am doing this in sections, when work and family allows, usually 7-14 days at a time, camping with the odd B&B. I am 49 and pretty unfit, so you don't have to be Lance Armstrong if you are not bothered how long it will take. I have done anything from 20 to 60 miles a day heavily loaded with front and rear panniers. I started from Southampton and have worked my way to half way across Hadrians Wall, so about half way, in a total of about 5 weeks cycling - might do Scotland afterwards, England and Wales for starters. I recently did the Yorkshire section and this was very painful hill wise, I could hardly push the bike up some of them, never mind ride, but if you don't mind pushing up the steeper hills it is generally OK. Because of this I am considering trying ultra light gear, but cost aside, most ultra light tents are like coffins, and I like my comforts like toast in the morning.
As for route, because I have no time limit I have chosen to do it using no A roads if possible. A mixture of Sustrans website and Google maps is great for working out routes and seeing what the road surface looks like. You can make out if a road is a farm track used by tractors usually or if a path is likely to be suitable by using the satellite images. You will have to come off the coast sometimes anyway to get around rivers for instance - never rely on ferries actually working, I suggest you call first as half of them are non existent, and often require a 50 mile detour, although I have found that boating people are pretty sympathetic so I have managed to get lifts across the rivers when I needed them.
I have really enjoyed my trips, and met some great people. I was a bit concerned about going through some areas such as the old coal mining towns of Yorkshire, but rather than being faced with gangs of drug ridden hoodies, the first thing that happened to me was that a guy in the pub offered to put me up for the night! The Yorkshire people, and pubs are my favourite so far, although they can stick those bloody hills!
Good luck, I hope you enjoy it as much as I have.