Unfortunately, weight loss is slightly more complicated than that, 450kCal/day for a month isn't quite 4lbs/month (even for fat) and my estimate of 350-450kCal/

Unfortunately, weight loss is slightly more complicated than that, 450kCal/day for a month isn't quite 4lbs/month (even for fat) and my estimate of 350-450kCal/day for walking several miles is likely an over-estimate. I would find it quite unlikely for most people to lose even 20lbs only as a result of added walking in 6 months without diet changes.