You wanna know why that one yakuza guy died? It was cause the yakuza didn't implement a rigid cardio/gym regimen for all its members.
But for real, do cardio if/when you can, and calisthenics if you can't lift weights. A YouTube channel to help would be Hybrid Calisthenics, and for cardio, bike rides walking with spurts of jogging, or even frequently walking would help.
Remember that it's incremental gains we're trying to go for.