Interval training for Taekwondo?

I think what the problem is here is that your body has become accustom to running. Cardio training is different for different things. In the opposite to your situation, I know a guy who was a wrestler, that couldn't run for very long at all, but in the mat he was a machine and could wrestle for hours (no exaggeration). You need to get back into sparring, because it makes your body work differently than running. Interval training would be good. Workouts exercising "fight muscle" is also good, as well as just getting out there and sparring more often. It should fall into place pretty easily and your skill level being as high as it is, you should pick back up to where you were in no time.