Happened to me today....I warm up with 6.2 and 1% incline for a mile and then alternate very 1/8th of a mile between 9.2 and 6.2 followed by a cooldown mile at the slower pace.  Now 6.2 is a pace I can maintain until my legs fall off. I've tested myself and can do 10 miles before I'm just bored stupid but can still maintain a conversation. Some fatty takes an open treadmill right next to me even though there's only 4 people in the entire row and proceeds to "run" at 6.3 lol. I was just finishing my warmup mile and ready to kick it into gear. As soon as I started the faster pace (still not a sprint), I see her slow to a crawl and then leave.