You can classify runners into two types: Associators and Dissociators. Associators find it most helpful and motivating to concentrate on their bodies and on actual running while they run - they say to themselves things like "Are my legs tired?" , "Do I need to pick up my knees more?", "Is my breathing laboured?", "I need to keep my head up" . Dissociators think about anything other than the run they are doing while they are doing it, so might think about loved ones, holidays, the surroundings, their running companions.
Identifying which type of runner you are can be key to making your runs easier for you. To some extent, you can plan what you're going to think about during your next run, especially if you've set yourself a challenging target.
Running is a mental activity not just a physical one!