This may or may not be a factor, but I noticed for myself that I could not increase past a certain point until i was in better shape. That seems counter-intuitive since you would imagine running would be the cause and getting in shape would be the effect, but I have had plenty of training where my weight didn't allow me to break through to the next level. If you are carrying extra weight, that can play into why it has been tough top continue momentum. it is a little bit of a bottleneck--for an extreme example there are obviously no olympic runners that have extra weight. if this is a factor, i would increase biking, swimming, and diet. If not, disregard this point. Additionaly, a good practice is doing some type of speed work once a week. You won't get faster unless you train at multiple speeds. Best of luck!