Think about where you struggle most on an Audax ride. With me it's the hills, and the cumulative effect of climbing them. I never have a problem covering flat ground at a decent pace.
A hour on a turbo is a long boring hour so I'm inclined to say, based on my experiences in Wattbike class, make the most of being indoors on a fixed bike and do intervals above threshold (zone 4/5/6/max). Doing Sub-threshold stuff is much easier out on the road then doing above-threshold training (My eyes are closed in Zone 5 & 6...it bloody hurts). It takes some will-power to build endurance on a Turbo trainer. I'd rather wrap up and go for a 4 hour Sunday morning blast than do similar hours on a turbo*
The gains you can make in a short time, using HIIT-type intervals, are incredible but it's almost impossible to safely recreate it out on the road.
My ability to hang on, on a climb, above threshold, has increased so much I can hardly believe it. I just don't run out of gas the same and it was that that was impacting my overall speed on an Audax, recovering from the hills or the headwind sections.
My heart-rate drops much more quickly, following a hard effort, than it did a year ago. It's getting used to hanging on, above threshold, for minutes at a time.
I'm basically a convert to the concept of threshold and >threshold training....where the pain is and where the quick rewards are.
*I'm not qualified to recommend any of this..I just go to Wattbike classes with a pro coach and am stating my own experiences of my own gains.