I've ridden the trail they use as a race course and it's rocky and steep in places. Harder than the upper parts of Castlewood at those points, about the same as C'wood in others, really closer to Chubb in difficulty. I wouldn't call those particular sections of the trail a beginner trail though my sister's done it as a beginner with little experience. She's tough but then again, so are you.

I have to admit that's pretty much the only trail I am familiar with out there and don't know what else there is or if this is the one they're riding.

Found this info, which you may already have, gives you some idea of what's what. Looks like only one of the options would be mountain biking. Will say 15 mountain biking miles in that area would be a challenge for you as a beginner, you're prolly looking at 2 1/2 to 3 1/2 hours actual moving time I'm guessing, you'll have to factor in stop times on your own.

http://www.parklandcyclists.com/pedal_the_parkland ....

There is an email addy for additional questions.