I’ve been running 1 mile runs and 5K’s
  I'm going to start training for a marathon by doing some half marathons and I'm already training my heart by doing heart rate intervals long distance. 
  I’ve used GW501516 and it helped me a little bit, but nothing too much.  Maybe i need to stack it with another sarm? 
 
Can you give me a good stack.  My current 5K time is under 20 minutes and i’m 27