Answer
12.56.. I think
umm you haven't said which part of the circles is shaded but I assumed that it's the gap between the two circles

54 stakes. 12" in a foot, 12x36 = 432, 432/8 = 54
David will finish first, because if you divide Brad's miles by 2 to his miles in a 1/4 of an hour are 5.95 miles. This means David went 6.2 miles in the same time that Brad went 5.95. Therefore, David will finish first.
You will have a better chance of winning because your chance before you opened the first door was 1/3 (since there was 3 doors) but now, you have 2 doors left, so it is a 50% chance that you will win.