While it is important to
gradually work up to a 20- to 22-mile training run in
preparation for a marathon, it is not necessary to turn your
legs into chalk dust during training...It is far better to reserve the long run for
every other weekend, or even every third weekend, and to
carry out high-quality efforts on days which used to be
designated for the long slogs.
...
Many marathon trainees believe that 18- to 20-
mile long runs prepare their bodies to handle the rigors of
a full marathon, forgetting that all they have really learned
to do is run a partial marathon at slower-than-goal pace.
To make your long training runs (the ones you carry out
every other week or every third week during your training)
relevant to the race, it is important to make such efforts
race-specific. This simply means including a significant
chunk of miles at goal marathon pace within the
overall run. You can be very progressive in this regard: If
your current long run is six miles, for example, you can
include three miles at goal marathon tempo (warm up with
two easy miles, cruise for three miles at goal speed, and
then cool down with one light mile). Over time, you can
increase the length of the long run by a mile or two per
workout, until you reach 22 miles – with about 10 of those
miles at goal marathon speed.
...
It makes sense, in fact, to complete one race simulator
about four to five weeks before your marathon date.
To complete the simulator, simply run 10 miles fairly easily,
at a pace about 45 seconds per mile slower than goal
marathon tempo, and then – without stopping – click off
10 more miles at goal marathon speed, before cooling
down with two miles at 45 seconds off marathon pace.
This great workout, which involves running close to half a
marathon at goal race velocity while already tired, is a diagnostic
one; it will reveal whether your chosen goal is
too lofty or too humble. It is also great preparation for the
marathon itself, since it forces you to reel off 10 goalspeed
miles when your leg muscles are already in a fatigued
state. Finally, the simulator improves confidence
and efficiency at hoped-for marathon intensity. Don’t forget,
however, that you must build up gradually to simulator
status, starting with about six total miles (with three at
goal pace). Bear in mind, too, that you will need ample
recovery after the simulator, completing only light training
during the following week and tapering your training
steadily and progressively between the date of the simulator
and your big meeting with “m.” As mentioned, the
simulator should be completed four to five weeks before
your marathon; if you squeeze the two together, you won’t
be fully recovered on race day, and you will not be able to
achieve your best-possible performance.

Jeff, I don't disagree with anything in that particular post.

But to me, any statement such as, to run an X:XX marathon you need to do speedwork, or you need to do xyz workout, is just not true.