The overeating/lack of exercise had to do with causes of morbid obesity in general.

I understand that this person has started to walk as a means of counteracting the lack of exercise, or are you referring to something else I may be misreading?

And yes, I understand that adjusting food intake is non trivial. How am I being glib? And how is that relevant to the metaphor?

Morbid obesity does not just spring up on you, your weight gradually changes and your eating patterns likely get worse. It is not at all like a sudden patch of oil.

It would be accurate to describe the situation in terms of a car driver not putting any maintenance into their car. Eventually the car starts to make strange noises. Later on still, the engine light comes on. As years go by, the car is driving slower and slower. Are we really surprised when the engine stops working altogether?