What the first three posters said. I do my winter base on a turbo for efficiency and on the road mostly because my locale is a mudfest in the winter. So with work commitment also in mind my build nearer spring is weekday mornings and evenings on the turbo then Z2/Z3 and other efforts on the MTB at weekends as my plan demands. By the time race season comes most of my riding is on the MTB.
With MTB especially, don’t ignore the more “all over” core and body fitness you need as well as the technical skills and position which makes riding your MTB all the more important. As above, incorporating small exercises on the way home is great - I’m gradually descending into a valley on the way back so often hammer tech section look for cornering improvements, or ride a flatter piece of single track with a “no brakes” rule to test carrying cornering speed / grow the size of my cajones.
Being “at one” with your equipment takes a lot of mental and physical stress away come race day.