Well, if you subtract time for all the necessities, you get about 15h per day left. If you take 1h of that for walking, that's >5% of lifetime, which is significantly more than reduction in life expectancy from being overweight.
Ah, I see where you're coming from. Agreed that it is time consuming. Perhaps one would see it worthwhile to make the time while they're losing weight, and then adjust once they no longer need to lose weight. The upside of walking is that most people will never be so sore that they can't walk every day.
That will eat your lifetime faster than obesity unless you also do something else.