Hi Robindobbin,
What you say looks obvious, but training is a question of balance and it is usual to do your long runs slower than marathon pace and some of your shorter runs faster than marathon pace.
The long runs are developing endurance, cardio-vascular efficiency (development of capillaries in the leg muscles and there for improved supply of oxygen and nutrients), and the ability to burn fatty acids as an energy source rather than the limited supply of glycogen that is stored in the muscles.
The long run has little to do with learning to run quickly. If you run all of your long runs at marathon pace the stress will be very high and you are likely to get injured.
Once you have a good long run base then you will see from the schdeules that as the marathon comes closer some of the runs get much quicker. This is where you teach your muscles to run fast. Some of these runs should be at marathon pace and some a little faster than marathon pace.
What you may want to do in some of your leter long runs - in the last 6 weeks before the marathon is to do what is known as a progressive run. Start the long run at normal pace but in the last 1/3 of the run speed up so that you finish at marathon pace.
This, along with the marathon pace runs, are good for what you can call muscle memory - once your muscles have learnt marathon pace then you will not find it difficult to slot into that in races even through you have't actually done an entire long run at that pace.
It is a matter of trust but it is a long established system that allows you to avoid overtraining by doing too much mileage at race pace.