Please help! I'm 20yrs old & have been running for 1yr. I did the london marathon this year (3hr 17mins) and I have been fine with my training up until 3wks ago. I've gone from running 35mile a week to about 5, I'm just so tierd when I run! My legs feel really heavy & after 20mins I'm tierd out! I have know problems with gym work, it's just running! Please help me!