Katty,
I just moved from Cambridge to Washington DC and I miss it terribly! If you are going to do the Transam with little biking experience you may want to ride down to London and back a few times to prep for the distances. There is a beautiful route that is about 70 miles to the centre of Blackfriars Bridge which is maybe a good training route? I know there is an annual ride from northern London (near the reserviors) every year for charity. Their route, ridden the other way round is a good one, and you can follow the canals from outer london dead into the heart of the city to catch a return train.

Also, if you start with the climbs of the northern tier, you may want to sprint up castle hill or the Gog Magog about twenty times daily to get your legs! Are you finishing Uni? Will this be a transam gap year?

Take care and have a cheeky pint for me.