How do you keep training when you have twenty minutes and no gym?

Twenty minutes done regularly beats an hour done occasionally, so the constraint is less of a problem than it feels. Structure that works: five minutes to warm up, then two or three compound movements in a circuit — squat variation, push-up variation, hinge or lunge, and a plank — for three rounds with short rests. That covers most of the body and gets your heart rate up. Keep the same handful of movements for six weeks and progress them, rather than doing something different every day, which feels productive and builds nothing.