Some days are harder than others. Today was one of those days. I used a slightly larger block of paper than my usual and a new chair. I just couldn't get far enough away from the painting to judge the composition. Some things worked, others didn't. But I've found that I always learn more by sticking it out.
Generally the stuff I do on site isn't as composed or purposeful as things done in the studio. There are more distractions, and I find I focus too much on the subject and less on the formal aspects of the painting. It gets more illustrative and less painterly sometimes.
The best thing about plein air painting is often just being out in the world. Letting things in that you normally wouldn't in the studio. Hearing the wind and birds. Noticing the light change as the Sun moves across the sky. Getting some vitamin D. Sounds corny, but it really is wonderful.
For me the best thing about painting isn't the end product. Its the process.