That is what makes the marathon so great, but it is also what makes the marathon so tough. One's genetics and training should also be taken into account. The same principle applies to those hoping for a sub four or sub five hour, just expanding the numbers a bit. That is why a little speed training helps, even in training for the marathon. That does not mean you have to do a formalized track session, just move your body down the road a little faster than it is used to every now and then.
Many marathoners simply hate to undertake speed work.
There is a reason they call it speed work and not speed play , you know. Instead, many folks run shorter distance road races. This is in effect speed training, if you run these races faster than you would have run the distance alone. These races will give you a very good idea if you are on track for your goal. That might be because that first six miles is almost all downhill.