At the end of every marathon training cycle any runner will be physically tired. Additionally to that the runner has to start managing the stress and the anxiety of his race day after working so hard to reach that last week, that last long run before marathon!!
If you ask any coach or if you see any training program from different coaches you will see different approach to that last long run the week before the marathon day. That long run must be executed carefully and for it works for one runner may not work for another. Always listen to your body and in case you feel really tired or you have tight muscles and you are afraid that you may injured maybe sometimes is even better to skip it and just rest. Usually the last long run before marathon is nothing else from a mental workout since is not possible the last week before marathon to gain any physiological benefits.
The ideal last long run before marathon day for me is the one that will reassure me that i am in a good condition and give me some boost that the Marathon Goal Pace i chose is the correct one. To achieve that i execute a long run that includes easy run pace KMs and some marathon goal pace KMs.
The long run session i follow is 20km (12 miles) by executing the first 10km at an easy run pace and finishing strong the session by running the last 10km on my Marathon goal pace and even the last 2-3kms even faster than that. Usually this long run before marathon boost me since i understand that 20kms (almost half way of marathon) are easy for me now and even the goal i chose for the Marathon is doable and during the run i register that this will be the pace that i have to go almost all the way for 42km the next weekend.
In the case that i feel my body really tired, the prior weekend to the marathon i skip the last long run and i just perform two small easy runs on Saturday (8k) and Sunday (8k) and on Monday i execute a 10k run with 7-8k on my Marathon Goal Pace to give to my body the feeling of going fast and in control in order to get that mental boost.
During the training cycle, though you have to go through different types of long run workouts that each one of them have their own physiological advancements. Some of the long runs you can add to your training cycle are: Long run for general resistance, long run for aerobic endurance, long run for aerobic power and Marathon Specific long runs.