Slower pace doesn't usually give me any more stamina because most of the energy is being used bouncing up and down.

The only mantra that seems to help is "I'm not here" or just counting to 20 over and over. As a general rule, positive self talk usually backfires and makes things worse.

As for mileage, I'm not doing any other running, but I do about 4 hours of CrossFit and 4 of Taekwondo, which has variable amounts of cardio. And for the record, I can hold my own on machine (rowing, bike, echo bike) cardio against the other people in my gym.