Yes, while you may not be able to restore your stamina of old you should be able to do something about your muscle tone. In fact doing so should help you with your health, your joints and even your FM.

Of course you have to be careful but even CFS patients who have much more difficulty than FM patients with exercise can improve their muscle tone and health with 'exercise'.

This first link gives some good reasons for FM patients to find a good exercise program 

http://www.vaxa.com/fibro-excercise1.cfm

The second link goes to a paper an exercise physiologist on that's on Exercise and CFS/FM

http://www.cfidsselfhelp.org/artcl_exercise_2.htm

The last link goes to some exercise tips on my website for CFS/FM patients. Notice even the CFS patient had biceps again after slowly incorporating the right kind of exercise into her day.

http://phoenix-cfs.org/TrtExercise.htm

http://www.cfidsselfhelp.org/artcl_exercise_2.htm

Good luck with your training program