I'm stuck on a grotty 50 mins. for my 10k (the last three races). I'm not too disappointed as I used to be rather tubby, but I do want to break at least 47 mins. so I've started doing some speed work. Intervals - 400/600m at abt 80% MHR. But I've read I need to do some fartlek work too. What is the difference between interval and fartlek and is there anything else I can do to build up speed over 50 mins. ... ?