Yeah, what used to work in our favor back in the "tough to procure food" days kind of makes our lives more difficult now. Our body is so good at staying alive it's pretty amazing. You CAN keep doing the same exercise, BUT over time you'll have to increase the intensity (for example if you walk on a treadmill you will have to walk faster or on an incline instead of on the flat) or the duration (5 miles will have to extend to say, 7 miles etc). But most folks get bored with the same exercises so they move on to different activities anyway.
"Giving up because of one setback is like slashing your other 3 tires because you got a flat."
"If you don't sacrifice for what you want, what you want becomes the sacrifice"