Today, 60 pounds lighter (at goal weight), I was heaving around a 42 pound bag of liter and thinking... 'geesh, this is heavy'... and then thinking 'this (... and more) is what I carried around with me for years' ~ every time I stood up, every time I took a step and even that first day of C25K. When I need a reminder of how far I've come; when I need inspiration to not lose focus, I go back to that bag of cat liter.
Give it a try, find something, anything that is 10 or 20 or 40 pounds and run up a flight of stairs a couple of times carrying your added weight. Then run up the stairs without the extra weight. Notice a difference?