When I think of endurance, the first thing that comes to mind is long-distance running or car racing. Endurance is certainly necessary in those areas, but it is of equal importance to freelance writers. Without the ability to endure--marathon writing sessions, inevitable rejections and the daily grind of finding suitable topics--the freelance writer is going to be hard-pressed to find success.
In writing, as in racing, pacing yourself is an important tactic to endurance and longevity. You may find you write better in short sessions done in intervals during the day; others may be more productive working in one or two long sessions. It doesn't matter which style you choose, as long as it's the style most suited to you and your situation.
You and I both know that there is productivity and there is putting words on paper. The latter of the two hardly shows your best effort. Productivity is when your mind and your fingers are in synch. That may not mean you're writing at 100 miles a minute, but it does mean that what you're writing has meaning.
Writing challenges such as this A to Z Blogging Challenge tests our endurance as writers. I hope to see you at the Z finish line.