Just saw this thread and I didn't read all the replies, so this might have been mentioned already. But here's a little insider info for anyone who wants to ride at White Clay/Middle Run: if you can, try to park at Middle Run. Two reasons:
(1.) It doesn't cost anything year round. In Delaware, parking at state parks costs something like $6 or something like that between March and November. White Clay is a state park, but Middle Run is a county park, so it's free to park there. The trail system is the same -- the best trails of the two are connected.
(2.) While the two systems are connected through the same trails, the "best" of the system is right off of the Middle Run parking area (the Lenape Trail and it's direct connections.)
And one last piece of info: if you go, make sure you hit it all! The trail system is flowy and fast and about as technical as a cheese sandwich, so you can do a lot of miles without killing yourself. It's worth it!