So I am a HSer, about to be a jr, training for XC. My problem is that I can run at a slow pace for forever but whenever I try to take it faster, my legs immediately get tight and my breathing quickens. The thing is, if I happen to stop and tie my shoe for 10 seconds in the middle of a fast run, I will instantly recover, and fell fine for the next 10 minutes until I feel like death again. Can somebody please help me out with this? Just ask and I'll give you any more info you may need to help me. Thanks.