6 Hours cycling, no problem. Mostly off road as well.
Using a bike is the best way to visit sites in and around the UK. Even if you drive to a point and do a 30+ mile loop.
If your thinking of getting a bike check out the BMBC for inspiration:
http://www.bristolmountainbikeclub.co.uk/
see my trusty steed here:
http://www.themodernantiquarian.com/browse.php?site_id=3436
See you on the trail :-) Gray Hill has some excellent bridlepaths to ride on.
Cheers,
Ike