I was training for specific distances a LOT longer than what I had done before, but pushed myself to get there. I also had my ex pushing me and telling me I could make it, and designed a workout playlist to keep me distracted and motivated.
Somewhere during that pushing of my boundaries, I discovered a point in my workouts where I felt AWESOME. Unfortunately, it takes me about 45 minutes of hard cardio to get there, so it's about 5 miles into a run.. but now that I've broken that boundary, I know that all of my workouts need to be longer than 45 minutes and then I'll feel amazing. So that's why I do it.
(this is still nearly impossible for me to achieve with swimming.)
It also helps to do activities that keep you mentally engaged or that are enjoyable in other ways - riding my bike is my meditation time, and it feels good, even if I'm working hard. Riding horses, I focus so much on what I'm training them to do that I forget I'm working myself until I get all done and realize how sweaty and tired I am. Playing a sport can be the same way - indoor soccer keeps me moving for a long time (I used to play a few games in a row in HS if teams after me needed another female), but I don't focus on how I feel as much as where the ball is and what I need to do to get it away from my opponent.
