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Without knowing how you personally respond to high volume indoor training it's hard to say.
Were you fit enough for long stuff in the past off what you did? If so, why not do the same?Flip side is, if you don't know, there's one way to find out... after the year then you'll know if you can you do lots of volume on the trainer without going mad and selling all your bikes.
Whatever happens you'll learn something :)
Personally I can't stand long slow turbo - I've done it a few times in the past and it does nothing except make me hate the turbo more (and that mental energy could've been used better elsewhere). Nowadays I want to bang out max 2hrs and I want it to be hard enough that I have to focus on it and such that the time passes and I'm "involved" and I can get off and do something actually fun, like sink piss in front of the TV (or count calories in your feckin' recovery shake, whatever floats your boat).
It's why I asked really... I just usually do 1-2 hour hard rides in winter outside then just do a load of low-mid effort stuff indoors. It's fine for every day riding fitness but I'm planning a couple of longer trips in early summer that I want to build for.
We're around 0 to -2 degrees here at the moment, will drop to -5 to -10 days in a few weeks for a month or so. The roads are usually clear enough for shorter high intensity rides but longer ones are problematic.