Pyschologists will explain this as Goal Gradient effect. We start to hurry up and put in more effort as we get closer to the finishing line. While doing push ups, try counting backwards 10... 9... 8... instead of the usual 1... 2... 3... and see how how it affects your motivation. Rats run faster as they approach a food reward. Humans increase effort as they approach rewards such as gift certificates or goals such as visual finish lines.