A Constant Pace Indefinitely - Measured Development

Software development has high and low points.  Likewise, there are hectic and "slow" times that we live with as part of our work-life.  When you think of software development as a marathon, a constant pace is a goal.  You want to be able to set a pace for the team that they can run at forever.  That allows for critical success factors like embracing change and adjusting to changing requirements.

Agile processes promote sustainable development.
The sponsors, developers, and users should be able
to maintain a constant pace indefinitely.

Burnout and similar situations are highly detrimental to a plan.  The loss of a resource or team in such an unexpected way causes slippage or maybe even failure.  There is nothing more difficult to manage around as a critical resource that is suffering burnout.  The recovery period is seldom something that can be estimated, and you effectively have "dead weight" on the team until they do recover.

It is easy for us to focus on our experiences when we evaluate processes and events.  That can lead to the "grass being greener" when we consider others.  Thus, in software development, we tend to focus on our part of the team as having a tough road ahead.  However, everyone involved in the project will have a similar experience.  It is easy to gloss over the toll a project takes on the project managers, customers, sponsors, users, and everyone else.  This process is not a marathon run only by the implementors.  It is important to note that the agile approach acknowledges the challenges for everyone and tries to alleviate them.  We are not solving issues for the implementation team alone.  These principles help lift everyone.

Running a marathon often includes hills, weather, and other conditions that help or hinder our progress.  These are important to acknowledge.  I think we can all agree that it is easiest to run a marathon in a controlled climate and a level and smooth surface.  This principle takes that concept and applies it to software development.  While we all know there will be challenges, we can still find a steady pace that we can maintain.  A little extra effort here and backing off there will provide us with a steady pace to take us across the finish line.  We can even have that final burst to finish strong.

The Twelve Principles and Overall Manifesto