Yeah like said before, there will be someone with the characters to take a photo for you, but mile marker photos you are on your own. You could do a "Self photo" just hold up the camera as far away from you as possible and take a photo. I did this alot on my first WDW Half because I didn't have anybody with me then and it worked out ok. I did have people ask when they saw me do this if I wanted them to take a photo for me, so you may find a friendly runner/jogger/walker who might do that for you.
Tracy