If you live in a relatively flat area, it might seem difficult to train for higher altitudes. But runners who can run at various altitudes (especially those who train at higher altitudes) are proven to be much more efficient and better runners. This is because higher altitudes, like areas found in the mountains and ares with higher elevation, have thinner air. Thinner air means less available oxygen to breathe. Obviously our lungs need oxygen to function properly, but so do all of our muscles. When muscles contract during exercise (i.e. what allows you to pick up your quads, put them back down, and propel your legs forward when you run) they need adequate oxygen. Oxygen carries iron through the blood stream to our muscles. Without it, we feel weak and can't perform as well.
Training for altitude involves finding a way to get in some hill repeats. If you don't have a hill near you to repeatedly run up and down in a timed interval (i.e. what a hill repeat is), then your next best bet would be to take to the gym and crank it out on a treadmill, with the incline set as high as you can get it. Running at higher altitudes also takes more effort overall from the body. So you need to mirror this when you train. That means training for effort, instead of just focusing on pace. Focus on pushing yourself physically to maximize effort. This might look like wearing light leg weights when you run or running carrying light dumbbells, or doing High Intensity Interval Training workouts to quickly get your heart rate up and push hard. Resistance running is great too. If possible, you can do this through pool running or using resistance parachutes.
Nutrition is important for altitude training as well. Because altitude running focuses on overall effort, you will definitely need to make sure you are getting in enough carbs so that you have readily available and full glycogen stores. And drink up! Higher altitudes will quickly make you thirstier for water than you might otherwise be.