IM Training Distance vs Time

Both are factors, because on race day you have to go a certain distance, but time is the more valuable metric.

Like most coaches, I usually give my clients training sessions by time and intensity rather than distance.  This is because our bodies only know time and intensity, and by referencing those, I can more accurately predict the amount of training stress that will result from the training session.  Distance of individual sessions is irrelevant.  Whether you spend two hours on your bike outside riding 40 miles at a given intensity, or you spend those same two hours on your bike on the trainer at the same intensity, but going nowhere, the total training stress was similar.

Of course, I'm designing sessions with distances in mind based on the athlete's goal events, but that's still the less valuable number.

There are also particular types of sessions where I use a combination of time and distance.  For example, track pace intervals.  The session may have a warm up based on time, main set based on distance, and cool down based on time.  But, the distances of the main set were determined based on the paces that I knew the athlete was going to run, and how long I wanted them to work during each interval and rest between.

You're on the right track by feeling that your recent marathon success was because of volume and not the long runs.  Total volume over time is more important than the long runs.

For marathon and IM training I usually cap the long runs at about 3 hours, regardless of how fast the person runs.  The time to recover becomes too long, and it can impact their training in the days after.  Instead, by focusing on consistency and total training volume, we build the durability and endurance necessary to race well.  Too many people tend to worry about individual training sessions, thinking in terms of what they did on a given day, when a better indicator of fitness and progress is what they do in a week or a month.

Using distance only can result in over stressing and inadequate recovery for slower runners, and under stressing faster runners, leaving them short of their potential.  Obviously, I'm not a fan of distance based generic training plans for slow or fast runners.  They can work fine for the middle of the bell curve, but often come up short for the people on both sides.

My suggestion is to avoid any plans that give only distance or only time.  Instead, choose one that uses time plus a well-defined method for managing intensity.  This can be heart rate or even RPE as long as the RPE levels have good definitions that help you train at the correct level.

This is excellent advice and pretty much what I was going to post so I'll just X2 it. 

In particular, the last paragraph - choose a plan that is time based plus a mechanism to measure/manage intensity.  Fink's IronFit programs may be a good choice; they are time based with intensity by heart rate / zones.

For measuring intensity I do like HR on the run and powermeter on the bike.  And field testing to set HR zones and the like.

From what you've posted about your experience, that's exactly what I'd do.  Assuming your experience with ultra distance running is deep enough that your "regular" runs each week are in the 1-2 hour range and average 10+ miles, 3-5 runs per week at that distance puts you at 6-7 hours and 30-50 miles each week.  For someone who already has that sort of running base, I'd focus on just maintaining run fitness and look at the best ways to improve swimming and cycling, so I wouldn't worry about 2.5-3 hour runs until the final build before tapering.  Instead, make sure you can swim well enough to come out of the water fresh with minimal fatigue, and build that bike fitness.  Running well during an IM is as much about bike fitness and bike pacing as it is about running fitness.  Undertraining for the bike or over-riding bike fitness, will reduce even the best runner to a walk.