Incidentally, one way to "ease" into the world of longer distance cycling is to sign up for a 12 or 24 hour race.

If I am not mistaken, with all of those events there is a set distance that needs to be covered in order not to DNF (something like ... 100 miles, give or take), but after that you can ride as much or as little as you want until the time is up.

Have a look at the 24-hour events on my Links page:
http://www.machka.net/links.htm

Also if I'm not mistaken, in England you've got the Mersey Roads 24-hour ... although I'm having trouble finding current information about it. The best I have at the moment is that the next one is being run on July 21, 2012 ... which gives you some time to prepare!!