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I don't do more than 2 harder sessions a week. I'm a high volume responder though which is why I can keep doing stupid long races without dying I think. Next year I'll be looking for speed rather than ultra endurance so my training will change a bit.
For 400k / day I'd be keeping up fitness over winter but I'd be looking to higher volumes in the new year - no point smashing out hard intervals now if your event is in June. You wanna just keep stuff ticking over now and then focus more towards the event. Specificity is high on my list of methods so I'd be looking at dropping turbo work next year and riding back to back 400 days to see how you cope, test your kit, kill off any discomfort issues, etc. If that's all sorted then you can move towards doing longer sweetspot and tempo work to bring your endurance/speed up. The faster you can ride your 400/day comfortably, the more recovery you get so the easier subsequent days will be (relatively speaking).
LEJOGLE, in that case, there are better qualified people in here to comment on what you are trying to do, @skinny + @hippy for starters.
If you are wanting to commit to indoors 2-3 times a week, I would personally consider following TR and doing;
Base: Sweet spot base, 12 weeks
Build: Sustained Power Build, 8 weeks
Both low volume supplemented with longer outdoor rides/audax events +commuting but ensure it's gradual and follow the rest weeks. As June is week 23 I'd skip a specialty phase from TR (and not sure any of them are event specific for you) but optionally do maintenance on the turbo then lots of outdoor volume.
That would be my take... happy for the long distance experts on here to suggest better approaches :)