The endurance runs are just that, training your endurance. If you try and run every long run (endurance run) at race pace, its like running a race every week which requires extra time to recover. You should be running your endurance runs at about 20-30 seconds slower than race pace.
Don't forget to pack your fuel and practice your hydration packs during your long runs. Your body needs fuel about every 45 minutes during endurance runs or anything over 1 hour. This is the time to practice for your race and see what fuel works best. I prefer Huma Chia Energy Gels.
Good luck and train smart!
Are you running longer than one hour?
Make sure you are taking in carbs every 45 minutes after you start running, otherwise you may "hit the wall." You also need to be carrying your water with you so you stay hydrated along the entire run. I try and remember to drink at every mile marker.
When you are training for a race that will take you longer than an hour to run, then you should be practicing with fuel to see which works best for you. A lot of people have GI issues and can't tolerate the Gu or Clif Shot Gels, but you need to try them all out to see what works.