Calisthenic strength, little and often.
Most training asks you to empty the tank twice a week. This asks for a few easy sets spread across the day, every day, and one steady hour on your feet each morning.
Strength is largely a skill of the nervous system, and skills are built by frequent unhurried repetition rather than by exhaustion. You practise four bodyweight movements at roughly half of what you could manage, always stopping with reps still in the tank. Because no single set is hard, nothing needs recovering from, and six small sets a day stack up to several times the weekly volume of two heavy gym sessions. You set the block length when you start, from six weeks to a year, and the whole programme reshapes around that choice. When a block ends it rolls into a fresh cycle, so it never runs out. Everything stays on the phone: no account, no server, no analytics.