John
It's actually also bridleway/RUPP which gave its name to the long-distance footpath. The Ridgeway is well known to mountainbikers. Although generally people do the fairly straightforward stretch from (say) Goring to Avebury. From Avebury the Ridgeway becomes the Wessex Ridgeway which is a lot bittier - although it regains continuity in south Wiltshire and into Dorset.
Oh, and it's kilometres.
Albal
I've been planning on doing it too. But if you live in Dorset you've probably got a better idea than anyone of the terrain!
If you need advice your best bet might be to try on
www.singletrackworld.com - I know there are some people from the Dorchester area who post on there and who would know the Wessex Ridgeway and be able to help.