Hi Lauren Your weight goals are good and so is your plan to run but 800 calories per day isn't enough. If you eat under 1000 calories per day your body will go into 'famine' mode. What happens is the body thinks that it is being starved so your metabolism will slow down. When you start eating a more normal amount of food again your body will store this extra food as fat, and although you lost weight quickly whilst on the diet you will probably gain weight fairly rapidly once you come off of the diet. The best thing to do is to exercise sensibly and try and stick to about 1500 calories per day. If you do this you will be getting enough calories so that your body isn't starving but you should still lose weight. Try not to eat the cookies and icecream too often but don't deprive yourself of them completely. If you completely deprive yourself of treats you will begin to crave them and this may result in you eating loads in one go.