Your body often notices the pace is unsustainable before your ambition admits it.

Ambition is bad at monitoring cost because its whole function is to keep you reaching. The body has no such bias, and it starts reporting the truth well before your plans are willing to hear it. Poor sleep, a shortened fuse, appetite that goes strange, a low hum of dread on Sunday evenings. These are not weaknesses interrupting the plan, they are data the plan should include.

Pick one physical signal you have been explaining away and treat it as accurate for a week. Adjust something because of it. You can always resume the pace, and you will know more than you did.