Endurance running for marathons is hard and gruelling. It isn’t uncommon for a runner to do 100km per week in the lead up to a race.
A good tip is to do 80% consistent pace and 20% higher intensity for both performance and injury prevention.
If you are constantly running at the same pace the joints, ligaments and tendons are challenged in the same range. It’s important to increase the pace and do some interval training to ensure you are stressing the joints in new and higher ranges to prevent overuse injuries.
Performance will be improved with interval training or high intensity training targeting the gluteals, Quadriceps, core and shoulders.
So for best results follow the 80 : 20 rule and challenge yourself to interval training and weight training to complement your endurance running.