I went to have a look round Rushmere today for the first time. Even after a bit of rain the newly built bits are still very, very soft.
I've not been to Rushmere/Oak Wood before so I'm not sure if any of the bits I rode at the Oak Wood end are new or not, most of it definitely wasn't as the singetrack was barely rideable due to the amount of brambles over growing it at the moment! Is the bit with the training drops and berm mentioned above new or is that old? I certainly seems to be in the area of gpx tracks I've seen on line that referred to the "Trek Trail"?
Personally I can't see myself heading there often as there's so much more, and so much more varied stuff over at Woburn. If you want to do a long ride then yeah, linking it in with Woburn would work but it looks to me (from map not experience) that it would be a lot of bland bridleways / roads to link the two. I'd rather just spend the time in the Woburn area as the singletrack is a lot more interesting.
I didn't see anyone else on a bike while I was there on a Sunday morning so on the up side you pretty much have the place to yourself!
I tried to get a pic of the map in case it's useful to anyone;

(Open the image link in a new window for a full size version)
I think Green = Bridleway, Pink = Walking path, Blue = Family cycling route, Red = "Trail" cycling route
Where I explored (covered most of the red trails);

A couple of pics;
Some of the new trail. Far left (in shadows) is the end of a decent, middle (to the left of the post near middle of pic) is a new climb. The other newly cut path on the right is a walker only trail.

Top of a decent (the one ending in the previous pic). Believe it or not there's a reasonable sized drop between me and the tree that the camera seems to have totally eaten!

Two small drops into berm - mentioned in previous post. Is this new or old?