Efficiency isn't the only factor. I'd say the most important factor is enjoyment. We each have a finite amount of willpower. If you're using it up on exercise you hate doing eventually you're going to hit a day where you don't have much willpower, skip the exercise and break the routine.

The most time efficient exercises I know aren't ones you can sustain for longer periods of time. I can consistently walk for hours but only run sprint intervals for a few minutes. Sure every minute of sprinting burns more than a minute of walking but if I can walk significantly longer than I can sprint it will probably lead to a higher total calorie burn.

That doesn’t matter when you’re sneaking walking into activities you’d normal do sedentary. It’s not competing against dedicated exercise.

I have this goofy plastic As Seen On TV board that is bent in the middle. Whenever I’m watching TV or playing a game, I stand in it and waddle my body so that i’m doing a light core activity.

It doesn’t matter that sprinting is better exercise when I otherwise would be sitting in the couch.

Depends on what the goal of the exercise is. For the majority of people who need to be in better health, zone 2 cardio is what they need rather than increased caloric burn. You can't look for time efficiency with zone 2 cardio - you just have to do it for enough time to build conditioning. Depending on the person, zone 2 might be running, but for most people a brisk walk is enough.

As others have said, it's not about efficiency. Also, doing a brisk walk does elevate your heart rate (mine goes up to ~100-110 BPM). If your goal is just being healthier, and not necessarily increasing your stamina or building up muscles, than walking is great. It also doesn't fuck with your joints.

> Walking is not great exercise from a time efficiency standpoint.

Sure, but it's infinitely better than a more efficient exercise that never gets done.