Why are you training for flat trails? Sure, they're out there. However, if you're racing a lot, some of the trails are going to have some climb. Don't throw your legs into shock. Prepare your quads and hams for ascension and your knees for descension by working in some hill repeats. That kind of conditioning will definitely speed up your flat game too. Watch Jeff Pelletier demonstrate hill repeats for faster trail running.
Jeff is training by doing 5 laps up and down Old Buck Trail (presumably the one in British Columbia).
Jeff and his crew start with a 20 minute warmup. Then, they tempo run up the trail for 5 minutes at Zone 3 effort (lactate threshold; should be hard to talk). They then run back down as fast/hard as they can and recover for 3 minutes.
After 5 laps, Jeff logged a total distance of 12 km with 500 m of elevation gain.
To get the same workout, Jeff says he would have to overrun on a treadmill with greater risk of injury. These hill repeats build strength while allowing both eccentric and concentric movements.