On my tail

Today, for a couple of miles, I had this chap hot on my heels. Perhaps I should be grateful, as in my keenness to keep good distance my performance was above my average (I'm sure). Why, though, could he not overtake me like everyone else? I gave him plenty of chances, but he didn't take them. He followed me all the way from Cheetham Hill to Prestwich - only when we hit a bit of a slope did he overtake and raced ahead.