You may want to do both (intensity and duration) but sessions when they get too long tend to stop giving maximal efficiency/ benefit. I'd go with upping the intensity and duration but after an hour just the intensity. Taking a day off will also help, if you train everyday your body is constantly fighting to repair itself. Taking that day off gives the body a chance to repair itself properly and build back stronger.
If you have your rig setup to do long indoor rides, they are far more beneficial in terms of fitness.
I made a rocker board for my turbo setup several years ago, got a large 4k tv for my crappy eyes, installed a sound system for music or link sounds from films. I have done 4+ hours on a turbo non stop pedalling. By far the hardest physical workout even compared to 8+ hours on the road
I agree about recovery days and variety to keep it fresh