Actually, I'd say that you can fake your way better through a marathon than an 800/1500, because you are not going anywhere near your top speed; you are just holding a slower speed for a long, long time, which is generally easier when you have certain types of injuries. My two best marathons took place in a year when I had calf injuries, one of them just two months before the marathon, and in both cases I couldn't do much speed at all. I was only able to do a couple sub marathon pace workouts each time. But I ran not too far from marathon pace or at mp on both occasions within a month of the races and that did the trick. With that injury, I never could have even tried to run an 800 or 1500 all out.