Good question, also interested in seeing answers.

List what you can do today:
5Km time
HM time
FTP
Can you swim 2 miles?
Can you bike 100 miles?
Can you run 15 miles?

IM isn't impossible by any means, and 12 hours is mid pack, so if you have trained in the endurance ability then you have a good shot.

Yes. Z2 speed.
Do 5-6 miles off the bike running at z2 hr. A lot.

Do a 3-4 hour steady ride ar z2 hr a lot.

Go as fast as you can at those hr’s.

Do a 2.4 swim every few weeks.

There is no good reason to do a 2.4 mile swim evry few weeks

If you mean there is no good reason to straight-through swim 4200 yards, I would agree with you.

That said, 4200 yards is really just a typical pool workout so I don't see why you wouldn't be doing at least one session in the pool of around 4k yards at least once a week.

Usually when someone posts doing a 2.4 mile swim, they usually mean straight thru. ..

Is this your first iron distance race?

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Pink? Maybe. Maybe not. You decide.

If you're familiar with QT2 at all they use their HR zones as a way to predict Ironman paces. In common zones like Friel (non-QT2) this would be using your Z2 HR.

Some things I've found helpful:
- For the run: Do your long run at stable Z2 HR the day after a big bike ride. Don't let HR go above Z2 unless on hills. If you add +30-45 sec to this pace, this will give you a basic idea of what's possible in an Ironman marathon on a good day (if you pace the bike well & nutrition is on point).
- For the bike: Same thing. Do some longer rides at stable Z2 HR and see what your speed is on similar course profile as your race. Don't let HR get above Z3 on the hills.
- The swim is a bit easier to gauge. Do some longer TTs and use a pace predictor to determine what's possible for the full distance. You don't need to necessarily swim the whole IM distance straight. A 1000 or 1600 TT will let you know what's possible.

Honestly man, I think this is tough to predict for what I call "front of the middle of the pack" folks, which includes myself. There are just so many things that can happen over the course of an Ironman day, some in your control, some only partially, and some not. For example, I went 12:59 for my first IM (Louisville) and was in MUCH better shape this year for IM Whistler. If you were to extrapolate the fact that my FTP and running paces were about 15% higher, I "should" have been able to run a 11:30 (which was my goal). So perhaps if you've done an IM before, you can use a similar methodolgy (compare times from some shorter tune-up races leading up to the event).

All that said, I had a VERY humbling day and barely jogged it in by dusk last year. The heat (my Wahoo says 105 degree high on bike) damn-near killed me. I doused myself with so much ice/water on the bike and run that my shoes were filled with blood from blisters. I have exercise induced asthma and forgot to grab my inhaler heading out for the run, or it fell out of my pocket, or who knows, but I felt worryingly short of breathe for about half the marathon.

So for what it's worth, my opinion is focus on the journey not the destination. Train your ass off. Stay in the moment on race day and do the best you can with whatever the day gives you. Have fun. And if you execute right moment by moment, have the requisite fitness, and get a little lucky, you'll achieve your goal.

A rule of thumb i've heard is to double a HIM time on a similar course/ conditions, then add an hour to get you in the ball park for an IM.

I'd think going 5:1X or 5:2X in a race early summer would indicate you are on track.

I've heard that a lot as well but I'm not sure if I agree.
IMO a 4.30 HIM person can go under 10h for an IM if all goes to plan, probably around 9.40-9.45? YMMV

Yes, this works if "everything goes according to plan." Which for me, never happens. But maybe I just suck at planning. I did a 5:20 HIM 3 months before my last IM, and went almost 15 hrs in the IM (vs my 11:30 goal, which was admittedly aggressive, but seemed doable given rate of fitness gains over those 3 months).

All,

Some good suggestions here. But I'm looking for a monthly checks to see that I'm progressing. This is not my first IM and I know that just going longer is not enough.

For example: FTP test every 4-8 weeks might be a good indicator. But I'm not sure I comfortable deduction from 1 hour bike to 6+ hours bike.

A 5 or 10k run might also be a good indicator but have the same limitaion.

On the other hand, a 3k swim is doable every month and can help ensure I'm on track.

Any other suggestions ?

First off, IMMT is a great race and a great venue. I've race there four times - two 70.3 and two 140.6.

In terms of benchmarks, swimming 3-4x a week is ideal, but hard for many to fit in - most swims in the 3-4k range will be fine with lots of faster 100's and 200's. I like to get in 3-4 open water swims of around the IM distance prior to race day, partially to dial in the wetsuit feel.

For the bike, I like to do lots of rides in the 3-4 hour range at 10-15% over IM goal wattage. If I can do a four hour ride around 80% average power I know that 70% on race day will feel nice and easy. It takes a lot of work to build up to those kind of rides though. Most of my Saturday long rides have a 30 min run off the bike at IM pace or a bit quicker. Some folks like to do a 'big training day' - what I've done a few times is a morning swim followed by a 4 hour ride, then an hour run at IM goal pace. If this can feel pretty comfortable, hopefully race day will go well. Also make sure you are doing a log of race day nutrition tests on longer rides & runs.

For running, I've had a few injuries over the years and don't really believe in the magic of doing a few 20 mile runs. Running 5-6 days a week and getting a bunch of 2+ hour runs under your belt will be far more beneficial than slogging through a few 20 mile runs when you are only running 3-4 times a week. Again, practice race day fueling on the longer runs - this may mean driving out and placing bottles or nutrition on your route or doing shorter loops to mimic conditions on race day.

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Hey mate!

Did my first Ironman in December at Ironman Western Australia in a time of 12:00:59

Happy to answer any questions you might have in terms of my training or what my benchmarks were!

^^^this^^^ Benchmarks for Ironman aren't across the board applicable. Everyone will be different. Knowing those benchmarks will come with training and racing Ironman races.

If you're just look for "fitness tracker" type workouts that also have a nice training stimulus built in, the three that I use are:
1) Swim -- After a nice warm-up, 10 x 100 at max sustainable pace sighting 1x.lap with great form, 10 sec rest, watch the clock and go. Look at average pace for the 100s and see if it's improving. Finish with some longer sets (4 x 400 at IM race pace), and see if you're able to maintain form after the strong effort 100s.
2) Bike -- 20 min FTP test is the obvious choice
3) Run -- This season, I'm try to race a 10K every 6-8 weeks (depending on if I'm on a 3 week or 4 week cycle). I find it's the perfect distance where I can drop the hammer and work hard, but it doesn't really leave a mark the next day like a half-marathon does. There will be some variability with terrain, conditions, etc, but in my case the trend-line has been obvious.

I maintain that unlike a workout like the Yasso repeats, there isn't really a single workout that can be used to "predict" you IM time, even racing a 70.3. At least at my level of experience (or with my bad luck), I find there to be too many race-day variables outside my control.

I'll give you an answer that takes a slightly different approach - but might help.

Assuming you are using a good training programme for an IM distance event you should be able to trust it to get you there in good shape (maybe not perfect but good.)
It should have repeat sets in the programme, you can use these as an indicator that you are improving.
If you are using a good programme and your performance is improving then that is a good process outcome.
Trust the process and let the results take care of themselves.
This will get you a better result for your ability and training volume than reverse engineering a race time.

Hope that makes sense.

p.s. IMHO IM race pace is done at what feels like an 'easy sustainable pace with a smidge of effort' in training. You can note this and see if it sits at your goal pace e.g. 180W on the bike, 5.40/km splits etc.