lately i've been running more, and i'd eventually (maybe a year from now?) like to do a marathon. i run 2 miles to the gym and lift, then run 2 miles back, and every so often i'll skip the gym and just do a 7 mile run. i set a personal goal, mostly for bragging rights, to do a 4:59 mile. i've gotten down to a 5:30ish but im wondering what i can do to increase my speed. are there certain workouts or exercises i can do to increase my speed and endurance whilst (almost) sprinting? or is it just something i need to get through repetition.