Thank you so much for the thoughtful and insightful replies. I have gained a lot of insight into swimming.
As I live in Taiwan, in some ways, February is a key training period as I will be racing IMTW March 19, IM LiuZhou April 1, and probably Challenge Taiwan full April 29. I’m also running marathons on Dec 18 and Feb 12. March 19 I hope to go fast, the others are for fun, and my “A” race for 2017 will either be IM Taiwan on Oct 1 or Kona Oct 14. (Trying to decide what my aspirations are for that race…went 10:01 in 2015. Have fun and enjoy the atmosphere, or see what I can do against the clock). Racing so often in Taiwan because a) it’s home and b) love racing with my buddies! (I guess a and b are the same!) BUT…I love swimming…and my training buddies just want to get out of the water and onto the real race.
My normal training time per week is 20-25 hours as I’m lucky to control my work schedule. Also, on weekends my wife trains with the same group, so even when we are running together, we are together, so that allows more time to train too.
I don’t have any aspirations for the marathons…but my friends have put the pressure on me to go sub 3…I’m 3:13 dispassionate marathon runner…but if I want to bring my IM time down into the 9:40s…a 3:37 IM marathon is not going to cut it. But swimmer’s physique versus runner’s…sigh 
So…based on what I’m reading here, it sounds like I don’t have the training time to sufficiently commit to swimming enough to do the race justice. So I think what I will do instead is do a 15km time trial in the 50m pool…cheap and convenient, and if it gets too real, I can drop out without having to be rescued. See how that goes, see how I feel, and then think about how much I want to do this and how I will go about doing it. 15000M of up and down should be a good mind test if anything else!
I am going through this dilemma EXACTLY right now, but my triathlon ambitions are less ambitious. I am coming off a 4 year rehab of my lumbar disc and finally have the ability to bike and run…yeah!
I am doing a 12km Lake crossing swim in Mont Tremblant in 11 days. Also I have started training for Olympic Tris at the same time. I have generally been swimming 100km per month since Jan 1 so my base swimming volume is there, but it is non specific to open water. Mainly I have been training IM strokes for the pool race season from Dec to May.
I think an endurance race is an endurance race. If you have volume you have volume…if you don’t have volume you don’t have volume. How you slice and dice getting to your volume is the key but you need swim volume. I think you need several swim weeks of 30km plus. If you do that while banging out some 24 hrs weeks of swim bike run, then I think you’ll hit it for your open water swim and for your tri season. Keep in mind that those 20+ hrs weeks of tri training while you bang out 30 km of swimming (I assume this is 7-8 hrs of swimming for you) is actually a huge advantage. If you are “just a swimmer” doing 30-40km weeks, you’re on a 10 hrs per week training plan. If you are triathlete doing 8 hrs of swimming and 12 hrs of bike-run, you will be pretty fit for the long race, probably fitter than the swimmer doing only 10 hrs of swimming.
I actually had a few “long days” that were 6km open water swim + 70 km hilly bike + 30 min jogging. That session is 4.5 hrs. I expect to be in the water for 3.5 hrs or so (ideally faster) for my 12km swim, but once I get much beyond 7 km in the water, the quality goes south…so better have multi sport combo days while keeping volume high…I THINK many of us have had success in open marathons the same way…run 6-10 hrs in a week and layer on 10-15 hrs more of swim-bike and we come out fitter than just an 8 hrs per week running program and go faster.
But I don’t think you can cheat the aggregate swim volume thing. You might as a lifelong swimmer more than someone like me who is an adult onset guy, but a long distance swim is like a 200 fly…50 fly you can crank out. The piano comes out in the back third in the 200 fly. I’d expect that a 15km swim is like that. Expect the piano on your back some time around 10km. The first 2 hrs of swimming come for free, just like the first 2 hrs of an open or IM marathon come for free…its once you go past 2 hrs in swimming or running that hell breaks loose. You need event specific volume for that.