Nice bike rides near but outside london

Dear everyone
I am dying to get out of town on my new bike for a day - i know i need to go and buy maps/ guide books and so on but does anyone have any recommendations for bike rides about an hour away by train from one of the mainline stations? ive spent a lot of time in brighton so not there, but i was thinking maybe north west of london but i really dont know whats up there and can only think of the M1. really dont want to be cycling in gruesome suburbia - which left to my own devices im sure id end up in - Im thinking fresh air and farms and so on. Id like to cycle from one station and spend about 4 hours on a nice bike ride to a station that will take be back to london.
My bike is pretty tough - so i think we can handle a bit of off roading if necessary.
Thanks!

I live in Oxford - precisely 1 hour from London. You can get from the station into the countryside in about 10 minutes.
There is some stunning scenery and nice empty A-roads running through the countryside. It's generally quite flat.