You can pick up a Garmin Forerunner 201 nowadays for about $120. It will give you distance and pace, accurate to within a few feet. It will tell you elevation change, show you how far behind or in front of your pace you are, give lap splits, it holds 500 some odd runs at a time, its all downloadable and logable to your computer, etc. etc... Oh, and it will tell you how to get back to your starting point if you get lost. An amazing little piece of machinery for just over a hundred bucks. I'd recommend it to anyone. There are some more expensive models that will whipe your a$$ for you, but the 201 offers the best bang for your buck.
Oh, and using different shoes is no problem with the GPS. You can even use it to bike or swim.