are you using trainerroad, by chance? if so, there is a trainerroad thread on hear somewhere that the trainerroad guys reply to. I used trainerroad some last year and this year and I'm actually following one of their plans for my HIM this year.

12 weeks out and you are just now thinking about building endurance?

Done HIM and Full before, working on my weakness :-)

It is always interesting for me to see the usual suspects, often not answering the questions, but yet taking the time to just second guess what people write and ask. It is as if there is no right question on Slowtwitch unless for the ones they ask.

Three months is more than sufficient time to build up and prepare so you cover the IM bike leg distance without distress. Like in other modalities, you can build up by distance or by time. I use long rides with a speed/power pickup at the end. I step up 30 minutes each long bike session (normally on weekends), keeping everything else as similar to the previous long session as possible. After four or five long bike sessions you will be pretty much at the distance/time preparation you would need so you can cover the distance on race day.

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After millions of years of selective breeding, here we are!

For IM when you need to run off the bike, I genuinely don't think there is any substitute for simply doing some long, steady effort rides. For mental ease as much as anything else, I'd always want at least a couple of century rides under my belt before toeing the line at an IM. If you're doing up to 8 hours across 3 rides, I'm assuming you've been doing a long ride of at least 3 hours or 50 miles already. If you've been doing that regularly you can probably jump up to doing a few century rides pretty easily in just a few weeks. 50, 70, 85, 100 progression in consecutive weeks should be doable if you're healthy and not too old. Time in the saddle on a trainer is also generally higher quality than the equivalent out on the road where you normally have to coast or stop occasionally due to traffic, hills, lights, etc. A 5 hour trainer ride is equivalent to a 6 or 7 hour road ride for me.

Also important to practice pacing and nutrition as well. A lot of people who hit endurance issues do so because they've not fuelled properly, or have been fluctuating the pace too much. The latter is less of a problem on a trainer since you don't tend to get power spikes like you do riding hills. Don't focus on endurance to the exclusion of everything else - I'm assuming that you do something like a 2 x 20 minute session once a week to work on FTP, that's really important to maintain.

If it's a bike-only event with no run, then I do think you can get away more easily with training on much shorter rides. I've done century rides in gran fondo or sportive events off the back of doing ~150-200 miles/week of commuting with nearly all my rides under 20 miles and a longest ride of only about 40 miles, and had no endurance problems. But running a marathon afterwards means you don't just have to survive the bike, you need to get off it as fresh as possible, and for me that means some long days in the saddle to prepare.

I haven't looked at the Trainer Road plan too much past the first few weeks, but for the half and full plans, does it ever have you in the saddle much more than 1.5 hours? I'm loosely following the HIM build plan on the low weekly volume. I'm still doing shorter races and weekend rides, so sometimes, I realize I'll have to veer off their schedule some. Just using it as a training supplement at this time. But I've not seen anything super long in the near future. Not too worried as I will still do some weekend riding like I said, but was wondering if the plan took that into account (currently away from my computer that has my T-road account downloaded)

I've only just started with week following their half ironman base mid volume plan. It only has a few rides longer than 1.5 hours. mostly 1 hour rides. My A race this year is a HIM in October so I decided to go through the entire Base-Build-Specialty plans for trainer road. I have couple of sprint tris and an olympic but they'll just fall wherever in my plan. My fitness level is fine for those. In the past, I've usually just done a 12 week focus plan for my HIMs because I usually maintain a decent base year round. This time I figured I'd give a full Base-Build-Specialty plan a good effort and see how that goes. With a lighter race year, it gives me something to focus on.

For the IM mid-level volume, there is at least one 5 hr trainer ride. A lot of 4-5 hour rides in the build phase. And suggested that if you do the ride outdoors, you tack on some time.

I just looked it up. For the HIM, there are some 4:15 rides in the high volume plan, but in the low volume, I think the most is 1:45 (which is pushing my ability to stay on a trainer without losing my mind in the first place). Again, that's the Half plan. I'm not touching the IM plan until at least November, and that's only if I feel there's a good chance I'll have the training time to go at a full next year. My weekend rides are in addition to TrainerRoad workouts. Typically low intensity rides to stay comfortable with handling the bike and logging time in the saddle/aero position. Every once in a while, I will also push workouts back in the plan if there is a race or big ride I have coming up. So I'm doing all the workouts, just not always exactly on the days they prescribe.