Skin Tightening Through Exercise or Weight Lifting?

While I do recommend regular exercise for general health, it is important to have the right expectations. It does not tighten skin. The degree to which skin shrinks with weight loss is determined by its natural elastic tone, which isn't as good for someone in their 50's as someone in ther 20's for example.
There are some non-surgical technologies which can tighten skin such as Thermage but the effects are not usually dramatic especially after weight loss; again, a matter of appropriate expectations. So do continue with your exercise regimen but don't expect to see the skin tighten as a result.

Dear Francy,
Given your age and the sagging you described of your arms, legs and buttocks, I think it would be difficult to tighten your skin through exercise and weight lifting. 
The arms are one of the more difficult areas to exercise.  You would have to exercise your triceps to fill out all that skin on your arms.  This would be very difficult.  In addition, you would have to exercise your quadriceps and your legs to fill out the excess skin in the inner legs.  Buttocks would require a significant amount of StairMaster but would also be difficult to fill out all the excess skin. 
Since you are agreeable to a body lift, I would proceed with a body lift which should significantly improve your abdomen, lateral thighs, lateral legs and buttocks. 
You should see how the scar heals and may realize that the scars are not as bad as you expected.  Hopefully the scars will be much more acceptable than having excess skin and fat. 
Getting rid of excess skin is always a trade off.  You have to decide whether it is worth getting rid of the excess skin and fat and living with the scar or putting off living with the excess skin and fat of your arms and inner legs. 
J. Timothy Katzen, MD, FACS

Morpheus 8 is a non-invasive skin and soft tissue tightening treatment that is consistently amazing in our patients. We use the Morpheus to exact fractional induced RF into the skin or tissue in order to tighten collagen and stimulate contraction of the tissue. For acne scars, it is quite useful to build collagen in patients who have atrophic scars but is not very productive for ice-pick scars which require TCA Cross. For tissue tightening, Morpheus is ideal for the face and neck and we often combine it with CO2 laser or Ultherapy to further enhance results. Chemical peels and skin resurfacing should be done at least 5 to 7 days after, but can be performed earlier, depending on treatment severity. Avoid exfoliating skin treatments before Morpheus and expect a period of 2-4 weeks for skin contraction to set in.

Best,
Dr. Karamanoukian
Realself100 Surgeon

Exercise and a healthy diet are great for your general health and offers a host of anti-aging benefits. However, this lifestyle cannot tighten your skin, especially when sagging is very apparent. You can try nonsurgical methods of skin tightening, such as laser skin tightening and micro needling, however results tend to be unpredictable and may require several sessions. Best of luck!