Most productivity advice optimizes output. Very little optimizes the ability to keep going.
Recovery is part of the work
Sleep, walks, and unstructured time are not rewards for finishing. They are inputs that make good work possible.
Design for the bad week
Every system should have a minimum version: one task, one walk, one honest check-in. On hard weeks, the minimum is the plan.
Measure sustainability
Ask whether you could repeat this month for a year. If not, the problem is the system, not your discipline.