A: If you’re walking with an added incline component (percent grade) along with these fast speeds, then you may be overtaxing the dorsiflexor muscles muscles on the front of the shin that insert onto the front of the ankle. If you’re doing too much speed and grade, you may want to reduce them a bit to get below your pain threshold, then add a bit more time. From there, gradually increase your speed and incline so that your body adapts slowly. In addition, you may find it beneficial to stretch your calf muscles since they resist the dorsiflexion movement.
Your pain could also result from overuse within a certain movement pattern. That is, the surface of the treadmill never changes. So your muscles get worked over and over through exactly the same movement patterns with little, if any, variation.
You mentioned that this pain occurs less frequently when you walk outdoors. This would make sense given that each step is a bit different than the last. The varied terrain forces your brain to recruit those muscles a tiny bit differently each time. For example,
Buy Authentic Cheap Cool Roshe Runs, if you’re out walking in the woods, you may have to walk up and down slopes or on slanted ground, so this automatically prevents you from repeating the same movement pattern each time. The variation keeps your muscles from getting "stale" and prevents repetitive overuse.
What about your walking shoes? Is your current pair worn out or lacking proper arch support for your feet? I suggest visiting a specialty running shoe store with trained staff that can address your concerns and help you find a proper fitting pair.