Remember this, your mind will quit before your body. The best most effective motivational tool you can ever have is saying to yourself "I can do this." This isn't just in the run, bike or swim, but anywhere in your life. This feeling of accomplishment, overcoming say a 10 mile run that you never once thought you could do will have you thinking "huh, I bet I could go further." Use that train of thought to overcome whatever life has to throw at you.
My marathon pace has me at just under 3:10 which means I can qualify for the Boston marathon. To hold that medal would be such a big honor. But again, it's not about time or distance, it's about the experience and what you learn from said experience.
This is what attracts me to the sport. It's a catalyst to bigger things in life.