how long have you had your shoes for? worn out shoes, imho, is the cause 90% of running injuries. it is fine that you land flat, that is your individual running gait. I wouldn't try and change it, you will just end up hurting yourself. on your next pair of shoes, get something with a lot a of cushion, and not a lot of motion control, because you are an underpronator.
Also, you could be overtraining. you have the cardio from running all summer, but you have given it a break for a while, which is enough time for your shin muscles to get used to not running. when you start back up again at your old level, you then get sore shins. try backing off a bit and then increaseing the milage bit by bit, that might help.