How long is too long for long runs?

This is pretty athlete specific.  It revolves around your ability to recover.  You don't want to blow your next few days out of the water because you can't recover from a long run.  My longest straight run this go through was 2:30ish and I had a split run that came out a little over 3 hours total. 

IMHO - Cut if off at 20m.  

There is a reason most every marathon plan stops at 20 miles for long runs.  Laws of diminishing returns starts to come into play after that.

I was in the same boat, a 3 hour run would stretch me into the 22 mile range.  I figures if almost every marathon plan out there has you only going to 20 miles, I wasn't going to go over that for IM training. 

I would personally stop at 18-20. Last year my longest run for IM was 16, 5 weeks out because I was injured. I would have liked to go farther because my past marathons I went between 20-22. My running training sucked for IM because of injury. Im my personal experience the IM was all about the bike. I spent lots of time with the bike and my conditioning level was fine with the running portion (would have been better if my ankle would have held up). Prior to last year running was my strength though so that might have had something to do with it.

My long run for marathon training is typically 22-23 miles, so that's what I do for IM training as well.  However, as noted above, I think this is athlete specific and you should only go so far as you can recover from quickly.

I think I'm going against common wisdom with this, but in general I think people tend to under-train for the run portion of IM.  Yes, getting in a ton of miles on the bike is important, but I think the reason so many people blow up on the run is that they haven't trained sufficiently for that piece of it and later blame nutrition, pace on the bike, etc.  I question the wisdom of a long run of only 16-18 miles when on race day there is 26.2 to go on tired legs.  Perhaps this is fine if the goal is "just" to finish, but it seems like more running miles are necessary if you want to race it.  I'm sure there are plenty of people out there who have proved me wrong, however!

I agree with this being somewhat athlete specific.  I generally don't agree with a marathoner's mentality being brought into IM training.  But what works for you works for you.

I don't agree with your second paragraph.  People don't blow up on the run because they're undertrained on the run.  They blow up on the run because they're undertrained on the bike, more specifically, they're undertrained for how they executed the bike ride on race day.  In general, people race like idiots and overcook the bike ride.

I'd agree with your assertion that people undertrain the run by only doing 16-18 mile runs if the blow up happens the last 8-10 miles.  The blow ups usually happen way before that which would be indicative of poor bike execution (which would include pacing, nutrition and hydration).

I've said this before and I'll say it again.  People tend to fixate on the marathon component of an IM when they should really fixate on the 112 mile bike ride.  The last 26.2 miles don't mean squat if you screwed up the previous 114.4.

I'm of the opinion that people overtrain on the run.  Not so much the mileage but how they get their mileage.  People overtrain the weekly long run.  It's often not properly proportional to their weekly volume.

I agree that people tend to blow up on the run because of pushing the bike too hard, past their training level. My school of thought is "race how you trained", ie, if you push past those limits by much you will suffer.

However, good run training may prevent them from melting down completely...but being cooked from the bike really messes up the run.

Thanks for all the sound and good advice my friends.  I am thinking 20-22 is going to be where I cut myself off on long runs.  Biking is my weakness and the last few weeks I have really been getting in some good long rides.  Plan on going out for close to 30 this morning with 10 of that being a TT effort.

Giving the new race wheels a spin as well.  Only have about 4 miles on them so I need to get on them for a bit and give it a go.

Again thank you everyone!

Were you also training to swim and bike while training for your 50K run?  That's the rub.  There's just not sufficient time in most people's schedules to train like a triathlete and a marathoner.  Sacrifices need to be made in the name of time and recovery.

If I had all the time in the world to devote to training, rest and recovery, I'd probably run 60-80 miles/week instead of 30-40.

You have to think cumulative stress.  You might feel great running 20+ miles but it might catch up with you later.  Personally,  its all about the weekly mileage for an IM and being fresh for the race. I would rather have lots of 15-16 mile long runs vs a few 20+ long runs.  Frequency trumps long runs.

Preaching to the choir.  I wish more people understood that.

My usual 4 runs/40 mile week looks like this: 10, 10, 10, 10.  Every two or three weeks I'll throw in a 16 miler for one of the 10's.