Cross country and track running are like any other sport, in that they both involve a lot of phyisics. When a runner goes out for a training run at 8 miles per hour, they will try to hold that speed for the whole run ( one to two hours.) This means no acceleration after the inital speed is reached. In a race, however, it is different. In, say, a three mile race, if a runner starts with a 5:25 mile, the goal is to accelerate every mile, and get faster as the end of the race gets closer. This seems counterintuitive, because the farther you run, the more tired you get, but that's how you get good times and beat other runners. When a runner gets home after a race or a hard workout, a different set of laws applies: a tired runner tends to remain at rest unless acted on by an unbalanced force ( the smell of food).