When you're riding your mountain bike you can’t really honk it in a big gear to get going faster. You need to stick to relatively easy gears so that you're always in a sensible gear to take on any obstacles that could be lurking just round the trail. This way you can keep a good momentum and take less energy out of your legs.
Stopping and starting all the time in a hard gear will soon take its toll. So, try doing the following exercises to make you pedal faster using the same gears.
Smooth not stomping
When you’re next out riding focus on where the power is in the pedal stroke. If you're fairly new to the sport, you're probably stomping hard on the down stroke. Imagine that each pedal stroke is a full circle and try to push and pull the pedal round as if you're keeping the pedal to the outside of the circle. Pedal in circles for two minutes at a time to start and build it up to 10 minutes, by which time you'll be getting far more efficient and be going faster without needing extra power.
Lift your knees
On another ride focus on lifting your knees faster and higher. This will eventually make you more efficient on the ‘up’ of the pedal stroke. Soon you'll be getting speed from an all-round pedal stroke not just on the down phase. Try doing it mid-ride for two minutes at a time and then build up to 10 minutes.
Up the revs
On another ride lasting over an hour do 20-minute blocks of pedal speed where you take your crank revolutions per minute up to over 100. Stay seated as you’re doing this. You may find to start with you feel like a clown and slightly out of control in the pedal stroke. But after doing this a few times you'll feel you're in control, with far more speed.
One-legged squats
It’s important to have equal power in both legs so doing one-legged squats will give both legs the chance to get stronger and you'll be able to notice any weaknesses. Try doing three sets of 10 reps on each leg keeping your abdominals tight to maintain good balance.