If you are determined to do both races, just know that one or both suffer a bit from being so close together, but you are not winning either, so what then? 20 hours a week is a lot of time to train, so there is no need for anything to suffer as you and others have put it. If it were me, I would swim 4 or 5 sessions a week of an hour, or 75 minutes if just 4 times. That should get you to just above 18k a week, and leave 15 hours to bike and run, which is what a lot of people have for all 3 sports.
Of course there is some crossover training affects, both ways too. But mostly it will cross over to your ironman. Maybe 3 weeks out from the swim race, do one week of 25k swimming, but that should be it, then taper off the yardage all the way to the ironman. The week after your big swim, you can mostly do bike and run blocks, with maybe two one hour swims in there. You will have to back off all intensities the week before your swim, and of course the few days after. Then you will have maybe 5 to 7 days in there for some good work outs, but mostly just staying healthy and uninjured.
It can be done, you have a lot of training hours, but the real point is that you will have to recover from those 20 hour weeks for it to be effective. The swim will be off by maybe 5% of what an all out effort would have been, and the Ironman maybe just a couple %, so the difference of having a good day, or a really good day…Go for it, just dont get all worried about either so much, as long as you get your work in…
I second what Monty said.
This is how I would break it down
Swim+ run focus weeks = 10 hrs swimming, 4 hrs bike intensity focus, 6 hrs runBike Focus weeks = 5 hrs swim, 12 hrs bike, 3 hrs run
Rinse, repeat
You will lose zero on your bike + run prep and will be aerobically stronger due to a high swim volume. In your IM you’ll get to T1 without even burning a single match and all that time in the pool will help your TT bike fitness. Alterating low volume and high volume run weeks and low volume and high volume swim weeks will help with injury prevention in both sports.
My only “worry” would be that your 20km swim race you’re going to be doing at a low intensity. I don’t know if it is 20km no wetsuit or with a wetsuit. I did a 12km lake crossing swim in 21C water this year and almost all my friends who raced no wetsuit got hypothermic just because of the time in the water and the low intensity…at some point if you are a 61 kilo athlete, you’re not generating enough body heat due to the low swim intensity and whatever heat you generate is being sucked out of your body just because you’ll be in the water for 6 hrs or so.
2 weeks after your 20km swim race is plenty of time to recover for an IM for a 20 hrs per week athlete.