Let's say your goal is to run 5 miles/day. The marathon is 26.2 miles so you would reach the marathon distance on your sixth day.
Cyclist? Try the Tour De France workout. It's 1864miles (3000km). Hey, even the pros take 23 days to complete this one! Simply keep a notepad and record your distance after every bike ride. See how long it takes you to cover the Tour De France!
Do you swim too? Try the Triathlon work out. 2.4-mile (3.86kilometer) swim, 112 mi (180.2 km) bike ride, and a 26.2 mi (42.2 km) run.
Not only does this work to keep me motivated it also helps me appreciate what athletes are training for and how hard it really is!