Hills!!!
Hills, they can make you or brake you. They can increase your speed. A friend told me long when I first started out 'Trail racing isn't about how fast you can run. It's about how efficiently you can climb up a hill or mountain, without blowing your self up and then how quickly you can get down without your quads quitting on you.' J. Meredith. I had no idea what he was talking about until I did my first trail marathon.
What I have learned about hills is divided into 2 categories of speed and efficiency. Hills can make you faster? Yup! Learning to get up and down a hill quickly without wearing yourself out is a skill in its' self. I started out by just trying to walk faster up every hill during my training. I didn't, and still don't for the most part, try and run up a hill it just wasn't effective or efficient. I discovered that if I can walk faster up a hill then running became easier and eventually faster. Then I read a book 'The Happy Runner' by David and Megan Roche in it they suggested picking sections of a hill and run it. Not long a long section, just before it might be too much. Once you got that down then start to add more to it, didn't have to be in the same spot. The idea was to just work on increasing your ability to run up that one section of a hill. The benefits of being able to run up hill are that one it's a lower impact on your legs, good thing. The other is when you're on flatter terrain you'll go faster, making it speed work in disguise.
Getting down the other side also has taken some work. Being able to run down steep technical descents is a thing unto its self. Go too slow and you won't really develop your quads in a way to handle the going down. Go too fast and you'll blow up your quads and be forced to either walk down or go down backwards. I'm still working on this part of my hill work knowing that long descents are part of it. I'm finding the balance between not fast enough and too fast, as the balance comes the speed going down hill will go up.
Hills and efficiency. I have a tendency to over do somethings and when I do it with hills it kicks me in the gut towards the end of the run. I've just burned myself out. Getting up and down a hill with as little effort as possible is a huge thing. A suggestion is to get to the point where you can out walk someone trying to run up a hill. If you can do that you'll out last them because you're not using as much energy. Less energy used on climbing hills means more energy for later in the race when you'll really need it. Being effective on hills will really add to your success on the trails.
Hills, you just have to get over them.