Dani recommended the dailymile.com site. I really like it. It allows me to map the route I want to take and let's me know many miles that particular route would be. This is a very useful tool. I used to have to drive the route to find out how many miles it was. This is so much easier. I find myself varying my route some, since I know that I can just map it out on dailymile and I'll know exactly how far I went.
Dailymile will actually let you know approximately how many calories you burn by calculating the miles you walked or ran, the time it took you to complete the distance and your current weight. So if you haven't used it yet, it's worth trying.