In the modern world with food abundance, it is hard to imagine that hunger has historically been the main motivation for exercise. Hunter-gatherers woke up each morning with the same goal in mind: to acquire food for the day. Off they went, walking and running, searching for meat and plant food to satisfy their hunger.
Nowadays, people exercise for different reasons: to maintain health, to lose weight, for sports, for aesthetic reasons. Yet all of these motivations can wane over time, mostly since they are not our "original" source of motivation.
It can be useful to "simulate" the exercise-for-food relationship. One easy way is to exercise right before a meal, with the meal being the "reward" for the completion of the exercise. For example, I often exercise after work but before dinner. Even though I'm hungry, I try to get out the door and quickly head away from the house. Once I'm down the road to a turnaround point, I naturally have no choice but to walk/run back home in order to get dinner. It's a simple idea, but it can be effective.