My problem is not having a good idea for what a long enough exercise session is. If I do something like 20 push-ups, and stop there, it feels like I haven't done enough exercise for the day for it to be any good. Then I end up doing no exercise at all.
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.
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?