The single biggest mistake in the example is giving an estimate for the whole project without knowing team velocity. He tried to plan everything up-front and committed to a completion date, strongly setting delivery expectations.

An Agile approach to this walk would have managed the expected target date much better and improved team morale by not overstretching each day ("sprint").

Lots of people think they know (and implement) Agile. I did, until a course I took about a year ago where I learnt skipping seemingly unimportant bits (measuring velocity, sprint retrospectives, etc.) bring the whole process crashing down.