K80,
Unfortunately, you cannot regain muscle without exercising. After mononucleosis you are not supposed to participate in contact sports. You can exercise.
Walking is probably the best general exercise for overall general fitness and cardiovascular fitness. You don't need any equipment, except for comfortable clothing and a good pair of shoes. You can start at any distance, increasing it just a little everyday.
Stationary biking is also a great overall fitness exercise. Again, start at any resistance and time, but just increase it every day or so, just a little.
Swimming or water aerobic are another great exercise.
Light weight lifting can focus on specific muscles groups that you want to strengthen.
Basically, you just have to start doing something. You want to get your heartrate up and tax your muscles, just a little bit. You can do the same exercise for several sessions in a row, and when it become fairly easy at that level, increase the distance, resistance, or time.
Getting back from illness and being sedentary is just like getting into shape to play a sport.
So, you might start with walking till you feel strong enough to take on some other exercise. But, you will have to be dedicated to doing the exercise. You have to exercise at least 4 days a week to build muscles and endurance. Less than that, you will mostly maintain what you have, you may gain a little, but not much.
Good luck.