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My Mum has fibromyagia, she's in her sixty's and although she doesn't run she does exercise regularly (sarock dancing). She does over do it now and again but she manages her energy very carefullly by taking afternoon sleeps/deep relaxation for 2 hours. She also uses some sort of electrical muscle stimulator.
It seems to take a lot of practice to judge the right amount to do and she often gets it wrong and over does things. Hope that helps?