Sometimes you never know

I'm visiting a friend this weekend, one who has always been athletic, fit and very gung ho. She was the first person I knew who ever did a triathlon. She runs regularly but has never done more than 4 miles, and lately 3 is her limit. I told her about my training and using a regular run/walk interval to extend your endurance. She'd never heard of that but was intrigued, so off we went for what we thought was a 5 mile loop. I was pleased to discover that I could keep up with her, and that my fitness was equal to hers. She lives near the shore, so we went out to the beach, and did at least 1/2 mile in the sand (including up and down the dunes to get out to the hard packed sand near the water.) We stuck to a run 5 walk 1 rhythm for a while, then switched to 4;1. I finished strong, and based our our time out I was sure we'd done more than 4 miles, maybe close to 4.5. But when we google-mapped it (I usually use MapMyRun.com) it was only 3.6! What a bummer, though with the sand part it was still a very challenging run! I should try to look on the bright side - I felt strong and comfortable the whole way. But with a 10k coming up in 13 days, I need to get more miles in! By the way, this friend is an amazing human being: this is what she does all day!