Walking is good for health and all the other exercises as well. But as Dumbledore said to Harry, the excess of even good things is toxic in large amounts.
She trained herself with the help of a local gym for a 10 km race and completed the race within two hours. While it was a feat in itself, it also had its side effects which turned out to be a little more extreme than thought.
Her story here is one of inspiration as well as of caution.