As you learn more about your running comfort (stride length, hand placement, breathing) you will be able to naturally run longer without loss of energy.
Some tips to conserve energy on a long run:
Feet Placement- The best way to expend energy slowly is to "skim" the ground as you run. (not literally, even though it feels as though you are.) When running a short distance people tend to run on their toes and bend their back leg high. Almost like a bounce. While this comes natural as you run, you do burn your energy rather quickly. Instead try staying a little lower to the ground and give little to no bounce. You will find that you can run miles more from this technique.
Midsection- Ever notice yourself twisting your ribs as you run? This can cause many problems for distance running. It can lead to the "side stich" also knows as side aches, shortended or lack of air flow, and shoulder pain. Try and keep your upper body square to the trail you are on. This also expends less energy.
Arms- Keeping your arms at a 90 degree angle and close to your side will provide momentum and comfort for distance. After 7-8 miles of straight running you may notice your arms and hands tingling. I shake out my limbs as I increase my mileage.