When I was 20-24 I worked a very harsh physical job, and I jogged or biked 4km to AND from work every day. But my diet was ridiculous, for example once a week I'd grab a whole loaf of bread on the way home and eat that along with some frozen chili. Or half a tray of lasagna, or a whole large pizza or something. I wasn't thin but I wasn't out of control either. At every checkup the doctor would mention I was overweight but didn't have any outstanding cholesterol problems etc. I felt great so I never worried much - full of energy, never got sick, strong like bear.
But once I quit that job to start college my weight and overall health deteriorated dramatically and it was very hard to get things back on track. I had forgotten how to eat like a normal sane person and those habits were hard to break.
I'm much better now in terms of diet and exercise, but it took a few nasty wake-up calls before I even realized how far I'd fallen. Like the first time I got back on my bike after over a year, and felt like I was going to have a heart attack after 5 minutes of pedaling. I didn't even look that much different yet, but I'd lost a lot of muscle and I was in horrible shape.