Varying the intensity on a random schedule. It prevents adaptation Syndrome. Break it up as to type as well. If you can't run, do some walking. Use elastic bands substituting for weights, and do the equivalent of low intensity strength training. Not a huge amount of resistance, just enough to run up the heart rate. That kind of thing.
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. “He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you.”- Fredrick Nietzsche
"We can judge the heart of a man by his treatment of animals." - Immanuel Kant