What are your goals? Is this for weight loss, weight maintenance, general health, to train for a race, etc., etc.?
I run/walk 6-8 miles per day, but I am training for a race right now. When I'm not training, I generally do 2-3.5 miles per day.
If you are new to running/walking, I think 2 miles is plenty. Just be sure that you are pushing yourself and it doesn't feel "too easy". If it does feel too easy, then I would recommend that you increase your pace or your mileage. You shouldn't feel like you could easily carry on a conversation if you want to be in a "fat-burning" zone.