There are a couple of muscles there that are deep to the tendon, tibialis posterior and a flexor of the foot. They could be causing the pain due to being unaccustomed at absorbing force and vibration (as well as the soleus and gastroc though you may only be feeling it where the tendon starts). Also your tendon may just be getting a little sore due to coming off of running which would go away soon if all else is right. The pic is right, the achilles is a long tendon but it is flatter or fatter (whatever you want to call it) up the top and then inserts onto the back of the ankle so it is not as flat then.