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As above really...have worked out Wendover not far away and have heard there's good riding around there..any suggestions?
Wendover itself is a small-ish hill so unless you session the trails or zig-zag a lot and have good local knowledge it's hard to make a 4hr ride there.
The Ridgeway goes through that area linking Wendover with Pulpit hill, Whiteleaf and Chinnor area - look on a map and have an explore with that as a basis. There's some GPX files on a local / Chilterns MTB club's site, based in Watlington I think - not sure as I don't use a GPS sorry but a google would find them. Also the classic Wendover / Hammer house loop has been in the mags a few times, an MBR killer loop etc, so if you can find that it's a good basic intro to the area, around 4hrs in average conditions.
Most of the routes you will find in guides are loops. But that could lead you to a lot of gloop this time of year, and you'll miss a lot of good stuff. The best riding, as Jameso says, is to explore/follow the bridleways from Wendover to Coombe Hill/Dunsmore, out to Great Hampden (tricky route finding at first), then past Chequers to traverse an excellent line across Pulpit Hill, with options to drop off the escaprement, then make your way to the top of Whiteleaf Hill. Plenty of options off that, and then you broadly retrace back, but aim to finish near the summit of Coombe Hill for a sweet final gully blast back to Wendover.