You either quit running or you run to you quit. The difference is your decision to quit, or your body's decision to make you quit (usually evident by the gag response). This will show you the uppermost limit of your anaerobic threshold. Once you find this, you will know how much more you can push during your workouts.
ha - no, man...that just helped to seal the deal with her. she's ready to join! probably be next month and she has some work/timing issues but we'll get her on board. thanks!