Usain Bolt acknowledged on Friday that repeating his staggering gold medal achievements in Rio in 2016 would be a tough mission as his younger rivals target his legacy. "I think when I get to 30 I will be thinking about retiring," Bolt said. "I'm not ready to retire yet. I love this sport, I have got all my success through this sport. I got all my fans through this sport. Track and field is way too hard. Yohan Blake is running 19.4 already, he's running 19.7, so in the next four years he's going to be firing. I think I want to get out before he starts running too fast." Blake, tipped as Bolt's natural successor as the world's fastest man.