No, you won’t “definitely lose weight”. You can easily eat more calories then you can walk off in 10-15 miles, so there are no guarantees. But if you aren’t eating while you are walking, and are being reasonable about your eating the rest of the time, you should lose weight be increasing your daily calorie burn by 650-1,500 by walking.
I love walking, and find it’s just as effective in keeping me from eating as it is for exercise itself. I usually listen to podcast concurrently.