Without seeing the training data you're putting out it's pretty hard to say why you might be 'losing' anything - could be a number of reasons. Top 3 guesses are sleep/lack thereof, you're overtraining and thus finding it harder to do faster stuff when you're trying to do faster stuff, or you haven't done top-end work in a while so it seems so much harder... But it depends, if you've got a coach what do they say?

*disclaimer: intervals done at above race pace are hard and you will run out of breath, people forget that!

For someone who is coming off of their off season & hasn't done much work on their top-end, the only recommendation is to do top-end work. It can be a mental thing and it does get easier. However if you've done a fair amount of distance work and you're looking at racing 42km, you won't have great top-end speed like a track star! Whenever I've struggled combining distance & tempo/threshold work, a few days of a lesser workload then getting back onto the proverbial horse by doing a very intense and short run workout.
Probably unrelated, but 80-90km of running per week is an awful lot of work your body has to do even at an easy pace...

Good luck!