I'm not having any problems with jumping jacks, toe-touches, squats, or sit-ups, and if that sort of thing was all I was doing, I think I'd have few-to-no problems with keeping myself on the routine of doing them.
The first day I started this 'get up and go' thing, I did one push-up; the next day, two; the next day, three; and so on. I'm currently in the twenties. I'm reasonably sure that what I'm experiencing is the 'burning sensation' you mention, though I'm not getting any sort of endorphin rush from it. And on the plus side, I actually /can/ do twenty-odd push-ups now, which I wouldn't have been able to when I started. (I know what that says about my physical state when I started.)
I've skimmed what free manuals and guides I can find, watched a few Youtube videos, and so on; short of buying a gym membership for professional advice, I think I'm doing things as closely to 'right' as I can manage. It just hurts, each day that I do n+1 push-ups compared to the previous day's n. (And, similarly, for holding the plank position for a couple seconds longer.)
Have you you tried doing shorter sets like 5x10 push ups with a minute of rest in between for example? You'll get much more push ups done this way, will progress faster and experience less burn. Try adding 1-2 push ups to those five sets every time you do push ups. If you reach failure point at any time, you're doing too many of them. Doing them every day might get counterproductive at some point, your muscles need rest to grow stronger. If you're already in pain when you're starting, you haven't recovered from the previous exercise.
"Cryonics has a 95% chance of failure, by my estimation; it would be downright /embarrassing/ to die on the day before real immortality is discovered. Thus, I want to improve my general health and longevity."
That thought has gotten me through three weeks of gradually increasing exercise and diet improvement (I'm eating an apple right now) - but my enthusiasm is starting to flag. So I'm looking for new thoughts that will help me keep going, and keep improving. A few possibilities that I've thought of:
Pride: "If I'm so smart, then I should be able to do /better/ than those other people who don't even know about Bayesian updates, let alone the existence of akrasia..."
Sloth: "If I stop now, it's going to be /so much/ harder and more painful to start up again, instead of just keeping on keeping on..."
Desire: "I already like hiking and camping - if I keep this up, I'll be able to carry enough weight to finally take that long trip I've occasionally considered..."
Curiosity: "I'm as geeky a nerd as you can find. I wonder how far I can hack my own body?"
Pride again: "I already keep a hiker's first-aid kit in my pocket, and make other preparations for events that happen rarely. How stupid do I have to be not to put at least that much effort into making my everyday life easier?"
Does anyone have any experience in such self-motivation? Does this set of mental tricks seem like a sufficiently viable approach? Are there any other approaches that seem worth a shot?