If there’s ever a time to seek the company of other runners, it’s for your hard workouts. Even paragons of mental toughness find they more often meet their hard-session goals by doing the workouts with one or more partners.
In addition to helping you get through the session mentally, workout partners can help you practice race tactics, such as tucking in behind someone else and drawing off her energy. You also get the physical benefit of others breaking the wind if you have different people lead different parts of the workout.
Doing workouts with others can also help you run more within yourself if you tend to push too hard in most hard sessions. Which leads us to ...