What pace to run long runs...

I have read articles extolling the virtues of:

Slow Long runs (60-90 secs slower than M pace)

M Pace long runs (how do you know you can run at M pace if you don't practice it?)

Harder Long runs (Dave Scott style training)

I guess most will do Slow or M pace long runs.  I am trying a bit of both with Slow 6 mile runs to aid recovery from M Pace runs.  I was reading about running harder but am not sure of the negatives.  I can see the potential benefits but surely the potential for injury is higher.   

Also I was looking at suggested HRs for marathons and saw a 2:20 forumite's blog stating 170+ HR for a marathon which suggests I am taking it too easy!  To race at this intensity maybe I should train this hard. 

The advice I'm intending to follow for my next marathon is to run the long runs at an easy pace (so around 60 secs slower than marathon pace) but I'll also add some miles at MP later in the run just to get used to running that pace on tired legs.

I will do some other marathon paced runs around the 10 to 15 mile range so that I get used to running at that pace,asthebig mistake in myfirst marathon was running too fast for the first half as it felt "easy".    

The HR thing depends on the individual, for me I reach 170 bpm in 5k and 10k races - but would kill myself if I attempted to keep that up over 26 miles!

All the runs you mention can have a place in a training plan.  A run that makes sense for one runner in the context of their fitness, ability and training plan would be suicidal for another.

a steady pace that you can maintain, ie work aerobically at.

no pain no gain is for idiots...

Your 2:20 forumite probably has a higher max HR than you. Mine's pretty high, but 170+ is ~HM effort, i.e. definitely not sustainable for a marathon.  HR is very individual.  (Having said that, elite or semi-elite athletes can sustain paces much closer to threshold than less well trained runners for marathon distance, so there is an argument for being able to maintain a higher average HR with improved fitness.)

Agree with Clearly - all the runs have a place in your training schedule but it's up to the individual to work out what's best for them.  For previous marathons I've kept all my (very) long runs at an easy pace, perhaps pushing it a little towards the end, but keeping my MP efforts completely separate, generally during a mid-week semi-long run.  Next time round I'll be making some of my long runs tougher by including extended MP sections or paces close to it, but only once the mileage has been built-up, and then certainly not on a weekly basis.