You don't say how you are travelling, but even if you have a car Devon & Cornwall is a bit ambitious for the time you have (understatement). many of the best areas are served by very narrow roads on which you can't travel too fast.
I agree. If yopu try to do to much, you will spend most of your time just travelling.
My experiences are with Devon rather than Cornwall but the roads are generally similar in width (or lack of it!).