You can do a ton on 3 hrs windows. I did a decade of IMs on 3 hrs window 5:30 am to 8:30 am on Wed and 6:30am to 9:30am on Sunday.
I was able to use two half days of vacation and convert three of those Wed morning windows into 5 hrs hard IM rides. And then I took one day of vacation to do a long IM simulation day (full swim, full bike, short run).
You don’t need week in and week out long rides. 3 hrs hard hammerfest at half IM race intensity for the bulk of it and you are set.
I’m def not qualified to answer anything IM-related (no IM history for me!) but I’m always amazed/impressed that folks seem to be able to knock out IM races with decent/good results for their ability on truncated bike volume, and just a few long rides thrown in closer to race day.
I knock out 2-3 hour rides at or near HIM pace every single week, but get me to do even a 4-hr ride and even if I slow down, things suddenly get a lot harder after that 3rd hour. Hanging in there for 5 would require some serious mental prep for me even if I can hammer the 3-hr rides weekly.
If you can hammer 3 hrs at 85-90% FTP you can go at a reduced hammering pace of 65-70% FTP for 6 hrs with zero training but you MUST stay strict to the max pace and you have to fuel for 6 hrs.
I can hammer at 85-90% FTP for 2.5 to 3 hrs on barely one bottle. Why. I already have 2 hrs of glycogen stored on my body. I can deplete that and on one bottle basically get enough calories to keep up the pace to the 3 hrs mark and end that workout on empty.
This is a false sense of security of using my 3 hrs ride to project to being able to last for 6 hrs. If I dial back to 70% FTP and I use the same fueling as my 3 hrs hammerfest, I will also be on empty at 4 hrs (can last a bit longer due to reduced pace). Keep in mind if you do NOTHING for 4 hrs you will get hungry due to your base metabolism alone, and this is a fixed overhead that you have to carry for the 6 hrs ride. You have to fuel for the extra workload and you have to fuel for the extra base metabolism
Here is some math.
Let’s say you have hammer a 90km off 1850 kilojoules
Let’s say you cruise a 180km off 3500 kilojoules (you should use less kilojoules to cover each 90km in a 180 than you do hammering faster in a standalone 90km due to the fact that you create more aerodynamic drag moving faster in the same aero position)
So you have to put an extra 1650 kilojoules to the road than during the 90km plus you have your base metabolism that also needs some feeding at some point. So let’s just say you need 2000 calories to survive the extra time (after the 4x conversion to calories and then dividing by 4 given human body efficiency of converting input energy to mechanical).
So there is your rough math (depending on your size) for scaling from 3 hrs to 6 hrs. Its more about pacing and fueling than fitness.
If you have 3 hrs bike fitness you have 6 hrs bike fitnessIf you have 3 hrs bike fueling you may not have 6 hrs bike fuelingIf you have 3 hrs bike pacing you may not have 6 hrs bike pacing
I did one 4 hrs ride in the last 5 years and generally my long ride is 2-3 hrs. I could get on the bike tomorrow and do a 180km ride without a problem. It would just be a gigantic pacing and fueling exercise. Almost nothing to do with fitness. The 6 hrs outing would just be a very slow version of my 90km ride (already slow), but totally doable.
Oh and my neck may not like it since holding up my neck for 6 hrs or 3 hrs is the same workload for double the time, It does not get easier just cause I am going at lower intensity, but it really is minor. And you or I would not be the only one. I saw Alistair Brownlee riding bolt upright at Waikaloa on the way back from Hawi to Kona at Ironman 2019. He looked like a BOP age grouper in that position, so yeah, even tough for the best. But the main thing is pacing and fueling