I'm a long distance runner and I mostly compete in road races 6 miles to 13 miles but I want to get much better at middle distance races and racing in the 2-5 mile range. I've been trying to train for this distance but I just can't seem to improve. I have a 4km race in 6 weeks.
I can comfortably run 10 miles with a pace of 7 minutes a mile or better but in training if I go even just a little faster than this lactate threshold I really struggle to keep it up even for short distances. I recently did 3 1-mile intervals in training with a 3 minute rest in between and averaged a time of 6:10. Some of the people I train with in my athletics club long distance group who I can do a better time over 10km+ with, can do a whole minute faster than me over 1 mile. I'm one of the better long distance runners in our group yet I'm possibly the worst in middle distance. Why am I so bad at running middle distance when I should be able to manage a much faster pace than I do and what can I do to improve?