I have a lot of difficulty in maintaining a faster pace (like a tempo or threshold pace) for an extended period of time. I recently have run 4:52 and 17:30 so I would say my tempo pace is 6 minutes. On normal runs I can hit 6:45-7:00 comfortably but whenever I drop under that (even 6:30 pace) it seems to get much more difficult and I can't hold 6 minute pace for too long-only about a mile. Does anyone know why this happens or what I can do to remedy it? I run about 45 mpw.