1. Without exercise, you are going to lose muscle mass. There are many more exercise routines then weightlifting. Please check "High Intensity Training" (not HIIT). Drew Baye is a good example. He also has a website: http://baye.com

2. Sustainable diet is the one which you can do, obviously :) I also will suggest a book, easy to read and will help you achieve your goals. The Lean Muscle Diet. Here is the Amazon link; https://www.amazon.com/Lean-Muscle-Diet-Customized-Nutrition...