I ran two times in five days with a friend. We ran 800-830 for easy pace for around 45-60 minutes. He felt fine but was a bit tired at the end. (Usually he runs 1-2 a week during the year with no workouts, but at school around 5 years ago he managed 12 minutes for the Cooper test). I thought I would take him for a workout because he seemed to handle the easy runs so well. We started out in normal pace, then we increased the pace to around 700 in a slight uphill. He slowed downed after about 1 minute of that and could barely keep running after that. I wanted to increase the speed after a while, but he said he couldn't handle. It took him about 10 minutes to recover. Then we was back jogging, although now probably 900 pace. However, when he tried to push the tempo, he couldn't even manage 800 pace. We then just jogged home around 900 pace.
How come he did so well for 60 minutes at 800 pace twice, then broke completely down after 1 minute at 700 pace in a slightly increased uphill?