I think that varies from person to person. Some people seem to be able to decide they're going to get into a routine and they just do it. Others seem to really struggle. Plus, starting a routine seems to be much more difficult than breaking it.

But the all-or-nothing attitude I mentioned above is problematic if someone has in mind that they're going to work out for an hour in the morning. For one reason or another they suddenly only have 45, 30 or 15 minutes for a workout and they write it off because they don't have time for their "workout" rather than using the time they do have.

But even if one only has 5 minutes you can make something with that time, whether it's a set of pushups, squats, mobility work or a quick jog. Then later in the day if one finds another window do something then.

IMO the best approach is try to move 5 minutes a day, everyday, regardless of the form it takes. Build the habit that you do something every day. Then build from there. Modulate intensity and movements. If you can do more do it, if you can't, don't stress over it.

You have to factor into time spent preparing, and transit. I'll give you an example. I like Brazilian Jiu Jitsu and there are 1.5 hour classes in the afternoon/evening. However, it's a 20 minute commute either way, and I usually get 5 minutes early an leave 10 minute after it's finished for stretching and cool-down/shower. So my 1.5 hour class turns into a 2.5-3 hour block every time I'd like to go to a class. Similarly an hour run before/after doesn't include transit, shower, etc., making it more of a 2 hour commitment.

Because of all the other things they have to “make an hour” for. Pick up kids, make supper, meh, I don’t to make a whole list, use your imagination. Personally, yes, that hour with a musical instrument in my hands is sacred, and dogs need walked for an hour whether I want to or not. But I’m not most people.

Besides, they owe neither you or me an explanation. Our hypothetical people said that’s not a workable option, what else ya got?

They usually can - if they have time for Netflix/etc - it's just not a high enough priority.

You don't even need an hour. 30 minutes does wonders. It doesn't need to include a trip to the gym either. Pushups, situps, squats and such go a long way.

I go at lunchtime. Stick a 90 minute meeting in your own calendar 3-4 times a week. Works for me 90% of the time.

Working hours and family commitments to name one.

Well, some people do, for instance me :)