Birdsboro actually connects to French Creek via the Horseshoe Trail. Basically, if you park in the main day use area of French Creek (near the ranger station), you could take Boone Trail through the Boone/Turtle combo and then pick up Horseshoe Trail about halfway around the Boone Trail loop. Follow Horseshoe out to Geigertown Road and cross the street to pick up the Horseshoe Trail in the Preserve. It is a long and steep climb up into the maze of trails in the Preserve. The "downhill-ier" trails (like Black Dog and Ramble On) are on the other side of the Indian Run Dam Loop. And if you head out across Haycreek Road area, you can pick up more trails both XC and (IIRC) the techies trail in the Preserve, Helter Skelter. There is a lot of up and down throughout the Preserve, but once you get away from Geigertown Road, it's nothing like the climb you do there. Lots of fun stuff, but it can be confusing if you haven't been there before.
Trailforks has a nice layout of the marked trails, but just be aware that there are a lot of unmarked trails in the Preserve as well. Some of them are among the best in the park, while others go nowhere and it's kind of trial and error to figure out which is which if you don't have local knowledge.
I've ridden there a bunch of times, but aside from the first time I was there, I've only ever done it by riding over from French Creek. And I am not yet totally comfortable navigating everything in a way where I could lay out a course. I usually start from Shed Road at French Creek, ride the Mill Creek section and then head over and take Lenape out around to Boone, ride all of Boone and then drop into Horseshoe to head over to the Preserve. That's a tough ride with a ton of climbing, but when you get into the flow areas of both of them, all the climbing pays off.
Definitely worth a trip, but if you want to justify the drive with a longer ride, I'd recommend combining it with French Creek - between the two of them, you can put together one very long, challenging day.