Best way to train for longer trail rides?

Is the best way to train for longer trail rides (right now I average about 2 miles, occasionally 3...and don't you dare laugh :P) by simply taking longer trail rides or is it a combination of my regular length trail rides plus some longer "endurance" type rides on flats or at the gym?

When I'm in the mood, I can ride around 10 miles in the gym...the last couple times I've chosen a hilly course - not hilly as in interval training hilly, but hilly as in I'm going up a hill for at least half of the tour - and I try to do it in a single speed (and not an easy one). But the gym is SO different from riding my bike...it's definitely not as hard because I'm not carrying the weight of my big ass PLUS the weight of the bike and my pack around. Not saying it's not good exercise, because on the days I can't get outside to ride, it's all I have.

So I'm wondering if it would behoove me to go out and ride something like Umstead State Park (that is EXTREMELY hilly and will kick my ever lovin' ass), which is not paved, but not dirt like a mountain biking trail...no obstacles in otherwords; or should I go for the American Tobacco Trail, which is a rails to trails deal where the steepest incline is probably 2%. However, the ATT kills me b/c of my tires (at least that's what I think). They are Kenda Nevegals and so are pretty knobby and dig into the sand/sifted whatever is out there on the trail...and so I get a good workout, but I can't get very far.

What should I do? How should I combine all these places? Pretty much I have 2 days during the week on which I can ride outdoors - and preferably on mountain biking trails...I'm obviously getting a good workout with the amt I'm riding since I've lost 12 pounds and many inches, and that's without even trying...no change in eating habits, etc. But since I'm on a roll...sort of...I figure I may as well get as much out of this as I can.