"I can run for miles and miles"

So...I am doing this training schedule from Runners World magazine that gears you up for a half marathon. It's 3 runs a week. One for speed, one for tempo, one for distance.
So Monday night I was finishing week 3 (a day late because I was busy pigging out on Easter weekend). Each week finishes with a distance. So this one was 10 MILES!!!
I sort of cheated because I had my friend Holly with me the first 5.8 miles, she is a new runner so I was running really slow and keeping her going. Then Ryan met up with me as she was getting tired and it was getting dark soo that I wouldn't have to run alone in the dark (awwwww, what a sweetheart). So I ran only about 4 miles or so with him (maybe I only ran 9.8, I don't know for sure). And that part was at a good pace for me (which is 10 minute miles on distances over 5k). But I certainly had a lot of energy because I wasn't even working up a heartbeat for the first 5.8 miles. But the distance is still the same so I couldn't have cheated myself that much! Tonight are my speed exercises, those make me want to cry.