How much vo2 training have you done?
It is something that is luckily very trainable, but if you dont focus on it you quickly lose that top end power.
You could try doing a few vo2 workouts over the next week or 2 and start to build up towards longer durations, starting with something like 30sec on/off @120% and eventually getting to 3 minute repeats at 120% and see if that gives you a good improvement.
I know its a struggle for me every spring since I dont really train that top end at all during the winter, I am trying something like what Chad suggested in this thread right now and the much gentler progression seems to be working great for me but that might change once I get to the 3 minute ones.