I apologize in advance for my english if it may sometimes seem a little unintelligible (it is not my first language).

I am experiencing difficulty improving my endurance for a kicking routine that is part of the belt testing. The kicking routine consists of 3000 kicks (a variety of kicks, 100 kicks per leg. For example, 100 side kicks per leg, etc.) in at most an hour. Kicks are either to pads or a bag.

The best I have done is 2000 kicks in 1:11 and 2700 in around 1:50. I had been training consistently for about 15 months and saw virtually no improvement with my time (for example, the time for the 2000 kicks fluctuates between around 1:21 and 1:15 on average, best time is 1:11). I used to run to help my endurance (best times are 6 miles in 52:04, and 3 miles in 22:46) when I was training more seriously. 

Before I started on martial arts I trained as a (sort of) powerlifter for about 7 years, so I suppose my current strength is ok (around 240 bench, 275 squat, 385 deadlift, 135+Bw pull up). I want to get back to training, but most of all I want to improve and stop spinning my wheels. Hopefully half the information here helps provide some background. I can give more details if they are required, thank you for your time.