I think I am getting into that old man skinny leg syndrome as well, but with me it seems that when I loose muscle mass it is replaced with fat.
What works best for me is a paleo ketogenic type diet and moderate strength training and moderate cardio. I find that if I overdo either one I start to put on a bit of chub around the gut. There is a scientific explanation for this that involves the production of insulin and the control of stress hormones.
I find that if I keep my processed junk foods to a minimum and get most of my carb from those foods that result in the least glucose production I can get away with eating a lot of delicious fat. For me what works is low carb, moderate protein and don't worry about fat.
My wife is vegetarian and she starts her day with a Costco protein drink with strawberries and blueberries added for flavor. For the sake of convenience I usually join her, but I feel better and stay in better shape if I make it bacon and eggs.
I do believe that if I would have adopted a healthier life style earlier I would now not be dreading the hip replacement in my future.
Good luck doing something about those skinny old man legs
