I’m also an MTB rider, but have started using an indoor trainer for a few months each winter in 2021. (Originally following Zwift training plans, but switching to TR last year.) And yes, your experience is normal - the first few bike rides feel harder than expected. I attribute it to a different pedaling position and much more varied power profile of unstructured rides (compared to TR workouts). I promise it gets better soon enough 
If you want to see your newly gained fitness, try a long-ish steep ascent. You will notice that you can now finish some ascents where you previously had to get off the bike and walk, and the ascents you could (just) finish have become easier. At least that’s my experience 