To get in shape after your muscles have lost elasticity and stregth is very risky. You can tore ligaments, rip tendons, or even unattach the muscle from its support. Any abrupt movements can cause permanent damage at old age. For this reason, exercising inside water is the safest solution. Water creates enough resistance to prevent abrupt expansion of your tendons.
Exercises that are based on sustaining positions and slow movements are also very good. Tai Chi and yoga are excellent. Being in my late 50s and still practicing abrupt karate, I prevent injuries by staying in positions that strengthen my tendons for about five minutes before I practice abrupt sudden movements.
Tendons, not muscles are what needs to be very strong. We must prevent tearing or ripping our tendons.