Depends how fast you walk, and what your weight actually is.
You could download the (free) Runkeeper app, and use it to track your walking, and it will give an estimate of calories expended (more conservative than some, which is probably better from the 'now how many extra calories can I eat?' POV)
It took about 40 minutes there (2.5) & 40 back (2.5) I weigh 10 stone 10lbs & im 5ft 3. I have tried an app before but it appeared to be telling me I'd walked & burned way more cals than I knew I had. Will give this one a try.. thanks.