Why guess? There are lots of tools to tell you this today. There are gadgets and there are free web sites that use Google mashups to create useful tools like RunningMap.
For example, what does 6 miles look like on the map? Well here is an example from my run today of 6 miles.
I've used this site for a while and I find it pretty handy. It is also good for comparing against my Nike + iPod to verify that it is calibrated. It is usually within 3-5% give or take but sometimes it is way off.
Sometimes conditions result in bad calibration though. For example, a run I did recently in the snow with my snow traction Yaks (rubber bands with metal that go on your sneakers) was off by 18%. That added up to 1.2 miles short that I ran. So now I know to add an additional 18% to runs in snow and to compare the run using RunningMap.