Recently, I was able to participate in and complete a Full Ironman triathlon. That is a 140.6 miles multi-discipline sporting event, consisting of a 2.4 miles open water swim, a 112-mile road bike ride, and to finish a 26.2 miles marathon-length run. One has 17 hours to complete this endeavor. To be honest, I really enjoyed it.
When you are on a training bike ride for several hours, have a few more hours to go, and you know you have to run after it, potentially in the heat, you really have to keep your mind in check and make sure that your mind doesn't race ahead of your body's current activity. It'd be futile to start thinking, 'Oh dear, I have three more hours left on this bike ride, ...