Missing a session. Does it matter?
The information below is in the context of Ironman and half Ironman racing.
Athletes with a high Vlamax (Anaerobic capacity) are much more impacted by missed sessions than athletes with a low Vlamax. If your Vlamax is high, your biggest friend is consistent training. Skipping rides will quickly undo the training and allow your Vlamax to start creeping up.
Athletes with a low Vlamax will be less impacted by missed training sessions (I'm not suggesting that you skip training). In fact, these athletes can sometimes get as much benefit by a short, super high-intensity interval session.