Only have the anecdata of being in both Tokyo and NYC, but obesity rates are clearly higher in NYC than in Tokyo. I'm not convinced that relative obesity in Japan vs US is primarily due to walking differences.

As another commenter posted, the amount of walking even a highly active person does is fairly negligible wrt weight loss/gain. As another anecdote - my ambient walking milage is 5 miles/day and I run an additional 20-30 miles a week and still maintain an obese BMI