At 48 I haven't faced that problem ...YET.
Of course I lead a active lifestyle and my occupation is pretty physical. Even so I recognize that I am not the person I was in my twenties (despite the fact I normally outwork most twenty year olds.

But can you regain what you once had? Simple answer is no. Father time doesn't have a reverse gear. However you can improve your strength and stamina. Just keep your goals realistic and attainable.
You also have an advantage. By now you should have learned to work smarter not harder. Use it.