The main trail is 3 miles, crushed gravel, multi-use (peds & horses). 1Â½ miles in, there is a cutoff to a doubletrack on the north side. That runs in a 2-mile loop. At the end of the first trail, there are cut-offs to two other trails, each about 1Â½ miles long. Both lead to the Walnut Hill Section of Valley Forge Park; one has a little under a mile of decent singletrack.
99% of this trail is strictly beginner - very little technical stuff, but there are long stretches where you can hammer for a few miles at a clip when the trail isn't busy. It's also a beautiful park with lots to look at, making it worth the ride when you just want to turn the pedals for an hour or two without a lot of climbing or stump jumping.
Two special notes:
1) There is another two miles of really decent singletrack hidden in the woods that are marked "no bikes," apparently reserved for the horsey crowd. I asked a park ranger if there was any good singletrack, and HE SHOWED ME THE EQUINE TRAILS! He told me that the rangers typically don't enforce the bike restriction unless they have a horse event or they get complaints about bikes running afoul of the horses. Rule of thumb, if you see a number of horse trailers at the Walnut Hill parking lot, stay off the marked portion of the singletrack. Otherwise, ride at your own risk (or ask a ranger first like I did).
2) This trail runs parallel to, and easily connects to, the Schuykill River Trail and the Perkiomen Valley Trail. Together these two trails are about 44 miles of mixed paved and unpaved surface that runs from center city Philadelphia to northwestern Montgomery County. While not strictly a mountain bike trail, there are definitely sections where a mountain bike makes more sense than a road bike.