I am training for my first marathon in November. I have been increasing my mileage carefully. Recently, I have begun to experience a bruised feeling on my left heel. It does not show any discolorization, and it feels like it is at the back of the heel just to the right of center. When I run it is sore, but after I warm up, it goes away, but returns after the run. This has been true for training and race runs. I have been stretching after each run, and as soon as I can, I ice the back and bottom of the heel with a plastic bag with ice.
Be cautious with this problem, it sounds like early onset plantar fasciitis. The plantar fascia is a thick tendon-like structure that runs along the bottom of your foot from your heel to your metatarsal heads. It is involved in making your foot into a rigid level as your heel comes off the ground during the gait cycle. We call the mechanism the "winlass effect". In effect, the soft tissues and the orientation of the joint surfaces allow the joints to "screw home" so that the foot transfroms from a mobile adaptor into a rigid lever for propulsion purposes.
Biomechanics result in your plantar fascia loading with greater loads than it can handle, leading to micro tears in the structure. As you begin to experience pain, the structure becomes inflamed (plantar fasciitis). Eventually, left unchecked, heel spurs result. Orthotics are usually very helpful in this regard, but make sure that the orthotist builds a plantar fascia accommodation into the device. Night splints help those more afflicted. NSAID's are a useful adjunct taken at theraputic doses as well.