The only way to travel
Bikes are just the prime means of transport in any built up area, you can reach speeds of 25 mph easily on the flat or a slight incline and with the versatility of bike lanes and multiple shortcuts I can generally beat any vehicle across a city. Plus I waste no petrol and the only fumes are those from the curry the night before.
My speedy bike of four years
Road or racing bikes are the key to success, they are quiet, low friction, high stiffness, highly efficient machines designed to allow the human rider to become one with the bike and travel great distances at great speed with minimal effort hence increasing endurance.
Man and bike, day in, day out, a fantastic experience. I rode from John o'Groats to Lands End in the summer of 2002 and what an experience it was. In total I did about 4 hours training but that was no hindrance at all, it seemed just so natural to get on the bike, weighed down with panniers, and ride south. There were four of us riding and we took fourteen days to complete the 990 miles that we traveled.
For me it rarely felt like a strain to 'keep going' and we endured just one day of rain and didn't get lost once on route (a couple of wrong turnings but that's all). Then when we got to Lands end I really didn't want to get off, in fact I didn't and cycled round for a bit longer.
My advice to anyone planning this is to go for it! Really totally worth it and if you ride daily anyway you should have no problems (though take some of those stretchy tube-grip things which are great). I would also seriously recommend going the opposite way to me - i.e. head North - that way you leave the rubbish Lands End 'theme park' and have all the exciting terrain ahead of you (plus the wind is more likely to favor you).