Running on a 1/4 mile track.... how far did I actually run?

I am going to run the army 10 miler in Oct, so I began practicing 2+ weeks ago. I have been practicing on a 1/4 mile track that has 8 lanes (4 lanes= 1mile.) I am now running two miles. What I didn't realize until a friend pointed it out to me is that I ran more than two miles because I didn't stay in the same lane for each lap that I ran. I ran in each lane so that I wouldn't loose count of how far I ran. Can someone tell me how far I actually ran? I think I may have ran 2 1/2 miles, but I am not good at calculating. Any great guessers, track coaches/runners, mathematicians, land surveyors out there? thanx