Do not despair. This happens when you get sick or injured and is a very temporary condition. You’re right that your temporary loss of fitness is due to the functional changes, mostly enzymatic and mitochondrial volume drop. Your right that you need to restore the lost basic aerobic capacity before expecting to feel like your old self and before introducing any intensity. My rule of thumb is that you do one day of low intensity Z1-2 for each day you missed. If you follow that rule I’ll be that after 7-8 days you’ll feel start feeling good again. Then you’ll need to address the loss in endurance by focusing on some higher intensity workouts as those will not have had 2+ weeks without any stimulus. But those too are mostly enzymatic and come back pretty fast.