Many studies showed that the Tabata workout protocol is one of the most effective (1) (only 2 rounds of 4 minutes) protocols to get back in good shape and sustain a good physical condition.
Here is one of the Tabata workouts that I am using every week https://www.youtube.com/watch?v=M4jNjZe9A5c
(1) https://jps.biomedcentral.com/articles/10.1007/s12576-019-00676-7