Thanks for your post. I’m glad you’ve seen such good results in improving your aerobic base. Your improvement in 6 months is quite normal for someone running as much as you are. Have you done this test yet? https://uphillathlete.com/when-to-add-intensity-training/
Several things may be contributing to your feeling more effort when you run fast. If your previous AeT pace was 10km/hr and now your AeT pace is 14km/hr it stands to reason that your high intensity running is also faster than it was before.
You’ve been training at what may be a slower pace than you used to train at. As a consequence your faster twitch muscle fibers (needed for fast running) have not been getting the training stimulus they were 6 months ago. So to engage them takes more effort.
I doubt it has anything to do with carbohydrate use.
I recommend doing 1-2x/week the following “pick-up” workout. Run as you would in a normal aerobic base run below AeT. During the run insert 6-10 x 10 seconds of faster running where you accelerate up to a fun fast pace for 10 seconds. This should not be a full sprint but a pace that can feel fast and relaxed at. Then slow back to easy running for at least 3-4 minutes before repeating. This will begin to prepare your legs for faster running before jumping right into harder interval type training. You should feel a good increase in leg strength from this within a few weeks. Then you can think of progressing to the following.
Find an uphill grade of about 15-25%. 1x/ week: Start with 100m repetitions on this hill with a slow easy jog/walk of 2-3 minutes back to the bottom between. Run this at a hard effort but not so hard that you can not complete 10 of these. After 4 weeks lengthen the reps to 200m. After 4 more weeks lengthen to 400m.
As for the different fatigue in your legs: It seems that it might be a running technique thing and you need to video yourself or have a good running coach watch you. You could try some single legged ME work like emphasizing the weaker leg.