Painting en plein air is quite a unique discipline. Going back to music terms it's a lot more like improvisational jazz than anything else. You show up and you might have scouted out the area first (I had, at a different time of day, making assumptions about what it would look like at dusk, which in this case proved correct) and bring with you the barest of supplies. Ideally you'll set up a bit early and have a little prep time to sketch out the scene if such is to your liking. Then you wait a bit as you watch the sun slowly creep along the sky, observing the shadows being cast and the colors change until just that right moment...then it's showtime. That right moment is typically going to be a little earlier than the time you wish to record for the simple reason that from the moment you start, you might have 2 hours, max, until the lighting changes so drastically that you will no longer be able to square what's on your painting to what is in front of you and are forced to stop.
It's a pretty hectic pace, and if you're working at evening, a bit stressful (in a good way). I kept looking to my right, noting where the sun was. I had to beat it--it was on its way towards sinking behind some mountains and if it got there before I was done, I was going to be sunk as the lighting would instantly disappear. In this situation you have to make choices quickly, what you will show and what you won't, the level of detail you'll put here or there, when will you commit to the shadows you are seeing though they are lengthening by the minute?
In the end, I had this small 6x8" oil: