Praise yourself and cheer yourself on for every 5 minutes of exercise you do! This may also help keep you motivated to keep going and doing more! You don't have to feel good about yourself only for the big things, but the small things too.
For example, when 5 minutes have passed, think about getting through another 5, and another. Before you know it, you are at the halfway point of your workout! It's the small things that count. Praise yourself as you go along- you deserve it.