What Can Be Done To Improve Heart Function And Stamina While Having Diabetes, Hypertension, Prior Bypass, And Very Low EF?

What can be done to improve EF of 25%. Age 72, diabetes, high blood pressure. Had 6 by-pass surgery 2 years ago and a defibrilator (1 wire) inserted 1 year ago. Could not go to cardiac rehab due to Covid-19. Not necessarily weak (for 72 yr old), get tired quickly performing any strenuous event. Can walk 30 - 40 yards before stopping to rest for approx. 1 minute.