I also do 3 hours per week of step aerobics. When I stand up or get out of bed I have some pain in the back of my heel.
Many problems cause pain upon arising after being seated.
The term for this is post-static dyskinesia which means pain upon moving after being still for a while. Pain before you start to exercise. In this specific case the pain at the back of the heel is not plantar fasciitis. Although the Achilles Tendon and the calf muscle tie in to the heel and can aggravate plantar fasciitis, since the plantar fascia is not located in that area the pain is not due to plantar fasciitis. Pain at this area could be insertional Achilles Tendonitis.