calories

OKay so i think i already said this but the board gave me 1200 calories. I am 50, 140 pounds and would love to lose at least 10, slowly! I work out every day using the treadmill at least for 45 minutes and if i dont do this i walk for 2 hours sometimes both, I also do so weight training. Today i went to 1700 calories and on the treadmill it said i burned 500 calories. did i eat to much or not enough. Some people say 1200 is not enough calories even if you do not exercise!