Hello!
The best step that you could take to prepare yourself for a tour would be to get in some cycling before hand. Try going for long day rides and also try to go on a 2/3 day cycling trip. Go somewhere nearby and dont push yourself. Practice cycling for a long time, so for example do 8 mph, but do it all day. This is the best type of practice as endurance is the main thing that kills. Go on weekend rides with the person you're going to be going on the trip with.
On the bike front a good idea would be to go to your local bike dealer, tell them your problem and they'll probably recommend a good bike. Failing that i'd recommend dawes (possibly the horzon) and raleigh (possibly the pioneer venture, a good entry level tourer). Ask around and surf the web on different bike sites.
Also you will want to make sure that you have all of the gear that goes along with a trip like this. Make sure you have good pannier bags that you can fit everything you plan to take into (make sure they're waterproof). Also you'll need the usual lights, puncture repair kit, coat etc...
For clothing it depends what time of year you are planning to go, in summer shorts, t-shirts, wolly jumpers, rainsheet etc. In winter you'll want long trousers that dry quickly (being wet will make any problem with the cold a much bigger problem). You can choose to buy all of the different specialist clothing, talk to the bikeshop again, but i spend a lot of my time just cycling in shorts + t-shirt that i got from ordinary shops (specifically legend).
You can choose how many clothes to take but you need to make sure you have a coat and enough jumpers to keep you warm.
The cycling gear i've got is-
1) T-Shirts- any random ones-various
2) Trousers-Tight ones that are snug warm- £30
3) Socks- Wooly socks that keep you warm even when wet- knee length- £10
4) Shoes- Used to wear trainers but recently bought shimano spd clip in shoes- Pedals + Shoes- £95
As you can see you can get a bit carried away
(i blame the sales guy)
Don't know anything about routes, maybe one of the more knowledgeable people can help you their.
Hope you have fun as that is what is always the point, just remember that people have done that route with £1000's worth of equipment and they have also done it with £0 worth of specialist equipment. You can have fun either way!
Good Luck- Post the reults of the trip!