Hill running is by far the best way to develop your trail running power, endurance and grit. Long hills are hills that will take you more than 6 minutes to run. Choose a steep but runnable hill. Keep running the whole way.
Run with big powerful strides. Keep pushing your hips into the hill. Maintain an upright torso and keep your core engaged. ( Read the article on Core Engagement if you need a better understanding ). Use your arms to drive the legs but keep the arms relaxed at the shoulder.
Heavy legs in the last 100 meters is an indication that you’re hitting the physiological target. Keep working when the legs are heavy, see how early you can get that to happen during the interval and keep pushing with those heavy legs. This is how you develop grit and confidence in your own ability.
The run back down should be relaxed and easy, you’re looking to recover your breathing before you restart. Walk the last 50m, in that time, set your intention, visualise the interval, especially the last 10m, turn at the bottom and go again.
There is also a mental component to hill training. You will develop confidence in your ability to run hard up a hill and keep going over the other side. Resist the temptation to take a break at the end of the interval. Rather roll straight into the downhill and use the downhill to recover your breathing and heart rate.
Keep in mind that this is a training session and running with big strides is intended to build muscle. On race day run the hills in whichever way feels comfortable for you which for the most part will be with smaller strides with most of the power coming from your feet and calves. (Read Racing the Hills to get a better idea of how to use your muscles on race day).