Obviously, treadmill. I run low intensity for long periods of time after lifting, and include some sections as fast as 8.7. This is to get in training with lower risk of injury, and also to prevent there from being a conflict between the gym workout and running.
I also include rowing machine, but what about specific strengthening of the muscles that support the knee, as this has been a problem for me in increasing mileage. Dead lifts and squats seem to do this best, lunges. Skipping rope as well, but I've been working out for general fitness there really, upper body as well. If you were to structure a gym workout around running entirely (including cross training and strength training to prevent injury) how would you do that? The facility offers a good assortment of machines and benches as well as weights. I was thinking trap bar deadlifts, and farmers walks would be most specific strength training to the range of motion used running.