Welcome to the forum and thanks for writing in with your questions.
I hope you feel like you have climbed out of the OTS black hole of despair. It’s not fun down there and some people NEVER recover. Since you’re coming back from OTS you do need to spend a few months rebuilding the aerobic base that will have gone away during your OTS imposed lay off. The aerobic metabolic pathway sees the fastest rate of decline once aerobic training is stopped. Luckily it comes back pretty fast as well for folks like you that have a good history of aerobic training.
You can never stop building or maintaining aerobic base if you are an endurance athlete. So there is no “Now my base is big enough I can stop base training” moment. The only real difference between base building and base maintaining is that in the base building phase (if you are aerobically deficient) you will want to JUST train at and below you AeT. As we have written in TftUA and here on this site You need to get your AeT to within 10% of your AnT in terms of pace or HR before you can consider your ADS cured and be ready to add intensity. In the base maintenance phase, where you will get after a sufficient base building period) intensity will be a regular part of your training.
During the base building phase we prescribe short (10sec) maximal effort very short hill sprints with full recovery (>2min) but no classic tempo workouts or extended speed work.
I hope this helps