No idea calorie wise, the only things I buy food wise is a sandwich and an oasis, I have the occaisonal twirl (<3) like once a week, when im at college at lunch, but that gets burned off walking uphill ot the bus stop, so to put it simply, burn of more than im putting in?
So you dont know how many calories your eating but you know that walking up hill to the bus stop will burn your sandwich,oasis and the occaisional twirl?
This is your problem straight away learn how much is in what you eat calorie wise. Twirl is like 255 cals (googled) for someone that is 16 stone it would take you 20minutes of running at 5mph to burn just the twirl off. A average sandwich is normal like 400-500 cals, oasis is 90 cals. Make sure you understand also how much exercise is required to burn cals.
Now I dont know the hill your walking up or anything but I doubt you would burn off as much as you think you do.
The main point is start monitoring what you eat and take into account salt,sugar,fat levels as well. Eat learn meats, dont drink to much milk. Break your meals up into 5/6 smaller meals aday. If you keep your metabolism going all day with small health balanced meals you will increase you metabolic rate.
Also I hate when I hear people say dont run because your over weight such a silly statement its not like every over weight person is running like usain bolt. The point is dont be stupid with your exercise start of slow and build up. Best way to do this is run/jog short distance until you cant run anymore then stop and walk the remaining part of the route. Create a route as well like 1 mile to start with I couldnt jog 100 yards when I got back into exercise. After a week I was jogging a mile your be surprised how quickly it builds up.
To many people will push them self to hard. Im not small I play rugby and have a knee injury from boxing of all sports and that was when I was skinny haha. I run now but Im not silly with how I do it. Run on grass if you can and get some good running shoes, running on pavements wont make your knees explode just take it easy built your body up to it while helping yourself to increase the amount of weight your losing.
If you are really worried and have a injury start on low inpact exercise like a cross trainer or something and move on once you feel ready.
Nothing will happen over night plan your weight lose slowly no more then 2 pounds a week.
Hope that long winded post was some help