I’ve seen that type of fitness loss / zone creep after a month off. What I do, and supported by my coach, is when there is that big of a fitness loss, is to ride by HR until power comes back.
General rule of thumb from the past two years, where I’ve consistently been putting down about 8 hours/week on average, is that:
- 1 week off will require about 3 days (Mon/Tue/Wed) to bring things back to where I left off. But I’m talking about creeping from 135-140bpm up to 140-145bpm in this scenario.
- 4 weeks off will require about 4 weeks, and I ride endurance by HR for the first 2 weeks and start to sprinkle in short intervals on week 2, and go longer on intervals in week 3, and by week 4 I’m nearly back to where I left off. Completely back after 8 weeks.
So the rule of thumb I’d use for a 2+ week layoff is to multiple by 2. Two weeks off = 4 week block to return assuming consistently training for a long time before then (and having been doing classic training with a lot of endurance). And 4 weeks off = 8 week or 2 blocks to return again given the same assumptions.
Jeff from NorCal Cycling YouTube channel uses that same rule of thumb.