If you're interested in how I keep going physically, the core of most workouts for me is Matt Furey's 'Royal Court'. He doesn't specify numbers of reps to do, or that kind of thing, and that's a good thing as far as I'm concerned. (Do I still do weight training? Sure. That's fine, although Matt doesn't any more.) But the Indian wrestlers, who called this dand-baithak training, do twice as many squats as push-ups, which means spending approximately the same amount of time on each.
The US Marine Corps instructors (who must be a lot fitter than me) do similar numbers in their workouts. My normal routine is: 250 Hindu push-ups; 500 Hindu squats; neck bridge (backward) static hold for 4 minutes; neck bridge (forward) static hold for 2 minutes; 100 ab crunchs; 25 slow side bends each side.
If you want good instruction on this, Matt Furey's your man. I came across his Combat Conditioning book in the now-defunct Sports Pages bookshop in London (the old grainy first edition of the book -- the new one is much improved). My first reaction was that some of the exercises looked too easy, while others looked too scary to try (the neck bridges). I'm so glad I tried 'em, though. Finding the book worthwhile led me to buying the videos, which add all the details you need to know, including the sort of cadence to aim for.
You won't find many good demos online, but this one's close to what I do. Except that unlike Mr Kurz, I don't train outdoors in the snow wearing just a pair of shorts.
Actually, my squat form is more like this...
No equipment necessary. 250 push-ups take about 15 mins. Likewise the 500 squats.
For a minimum routine (or a warm-up to other training which is going to be intense in its own right) I do: 100 Hindu push-ups; 200 Hindu squats; neck bridge (backward) hold for 2 minutes; neck bridge (forward) hold for 1 minutes; 100 ab crunches; 10 slow side bends each side. The whole routine takes just over 15 mins and takes no equipment and little space: a great whole body workout for the busy person, even in hotel room.
If I'm doing bag work I'll do either the long or the minimum routine, but without the abs work (since ripping a couple of hundred roundhouse kicks into the bag does the job nicely).
The straightforward protocol is straight sets. The other useful way is to do 10 sets of 25 push-ups, 50 squats then some other exercise -- skipping rope, sprinting, shadowboxing or even bag work (try 1-minute short rounds). Most excellent stuff.
Mens sana in corpore sano, Pilots.