Teaching the body how to switch gears is important. Both up and down.
There are always times within a race whether climbing a hill or passing someone that you will be required increase your power. This will inevitably cause the heart rate to go up and the breathing to increase. Typically, this could also be a period where you begin to feel uncomfortable.
By doing speed intervals (Fartleks or whatever you label them ) and then slowing the pace to 80 – 85% of normal you are teaching your body to recover on the fly. This is important because you don’t want to have to walk every time to you run up a hill or increase your speed slightly. ( There are certain times this is acceptable, but by in large we try to avoid )
It can also help you learn to handle that uncomfortable feeling of an elevated heart rate.
These are things you can do on your own during your long runs with no assigned times. Pick your pace up slightly..and then bring it back down. On a hill, run through the top of the hill…don’t slow down at the top.
Cheers and Run for FUN!