In order to improve cardio-endurance, workouts need to last at least 45 minutes.
Do hill workouts to condition your legs. Do 1 long run of 10-12 miles on a hilly course a week. Find a hill about a 1/4 to 1/2 mile long and run up it about 8 to 10 times at 75-80% effort, take it easy on the downhills. If you live close to a trail that goes up a steep hill for a 1000-3000ft, carry a heavy pack, loaded with water, dump the water at the top and cruise back down. Workouts that simulate what your objectives are seem to work best, but good overall fitness is helpful.