Rather than copy & paste it all, here's my latest blog which is all about the key to marathon training, heart rate based training. It's from my running coaching site, Run Well Marathon Training;
The longer you go, the more important is your aerobic energy system.
http://www.runwell.net.au/blog/2018/2/11/key-training-for-marathoners
By training at your aerobic threshold, you can maximise your aerobic energy production, thus conserving glycogen for later in the race when you need it most. The less glycogen you use, the more efficient you are.
