30 minutes/day has some evidence behind it if long term health outcomes is your main concern. I don't recall if 3.5 hours per week broken up in different ways than daily is just as effective.
A somewhat old fashioned circuit of walking for 30 minutes interspersed with a few pushup and pullup sets is certainly going to be helpful. You sometimes see paths at parks purpose built for this with exercise stations scattered around the loop.
Yeah, that's a problem. I can imagine myself doing 5 or 10 minutes of calisthenics at home, but not 30 every day. The exercises (push-ups, squats, burpees, kettlebell stuff) tire me pretty fast, and get boring fast.
I can manage 30 minutes jogging, and find it fun, but I worry that my knees will break up if I do it too often. They hurt several days after a weekend of running a 5K and hauling stuff up flights of stairs couple months ago.
With Alicorn's permission, I'm resurrecting this thread.
I'll start off with one of my own: What kinds of exercise can I do at home (I do have 5- and 20-pound weights), and what are good ways to get motivation to do so regularly?