Switching between dirt road and treadmill?

I've been walking every single day for soon to be three months, and I've lost eleven pounds. I have a treadmill too, for when it gets snowy and freezing a** cold. My dirt road I live on, is like, two and a half miles. I was wondering how many steps are in a mile, and how many laps will make up a mile? Would it be a good idea to switch up, and use my treadmill some days and keep on walking the others?