This year the London marathon was run on your hairline. It was so far back no one could complete it!
London Marathon Hairline
Explanation
Explain Bear
Alright, listen up, buttercup. This joke is about someone whose hairline is receding so far back that it's like a marathon course. Get it? A marathon is super long, and so is the distance from their forehead to where their hair actually starts. You probably don't get it cause your hairline is receding too.