I started walking 2 miles a day, it takes me about 40 minutes

... I don't know how I will do this when it's hot and humid out. Does anyone know how many calories this is burning?
I have lost 11 of the 17 pounds I gained since quitting smoking, but I've cut back on alot of the bad foods I'd been eating too.
Is anyone else exercising? This is my first real attempt at exercising in the last 15 years or so.
Another 20-25 pounds to go.