Thirty minutes on a treadmill or reclining bicycling seems a lot of time but if you set the display to time elapsed until you get to 15 minutes and time remaining for the last 15 minutes it just doesn't feel like much. After all it's only 15 minutes!
I know this is a head game but sometimes it helps you to keep on going. "I only have ___ minutes to go." or " I've only been exercising ______minutes." Breaking it up mentally into two segments just makes exercising more manageable.