Dehydration also has lots of side effects associated with it and headache is one. A significant one. If we couple that with the hard work you do and muscle strain?
It sounds like migraine headaches to me, and not as severe as most. Migraines begin with vision disruption as you describe it, then an incredibly bad headache, and nausea -- just what you're reporting. Anyone can get them for different reasons. These headaches are caused by vascular disturbance, when the blood vessels constrict and then open up. Unfortunately, they open up in the head first and at the extremities later. Not eating enough especially on a hot day and then exercising can bring them on. Certain foods can bring them on. If you have a period of stress, when you relax that can bring them on. I'm not discounting dehydration or blood sugar, both of which can also bring on a vascular headache. The only reason I'm wondering about that is that you were pretty used to the routine you followed and hadn't gotten this before. I would also say, you're getting older all the time, and as we age we go through stages that can come on suddenly. At 37, I could play full court basketball for 3 hours and keep up with anyone. At 38, I couldn't keep up anymore. Stuff happens, bodies change. At any rate, you have some stuff here to talk about with your doctor. There are non-medication ways to deal with all of these problems that your doctor probably won't know anything about, so don't think the doctor visit is the end of it. For me, meditation and some natural remedies got rid of my migraines pretty much. But the first one I got, living in LA, was on a hot day, I didn't eat much, and I went out and played basketball. I was 13, and got them until a doctor had me start meditating when I was in my twenties. Can't tell you if I'm right or not, just offering a possibility. And know that every year you get older the better your nutrition and sleep should be if you're going to be doing such hard labor and not end up with some chronic injuries. Which is another thing -- I would expect if you were dehydrated you would be suffering muscle cramps, but you're not describing that. Good luck.
This indeed does sound like a combination of dehydration and low blood sugar. You have a physically demanding job when in the woods and in hard conditions. You are like a professional athlete with a training session all day long! Think of it that way. You have to make breaks for food/water.
I would go ahead and talk to your doctor about this even if intermittent. It's always wise to get a full physical especially with your activity level and these episodes.
Set that up and come back and tell us what they say! (and even though you were writing for a difficult reason, I enjoyed reading about your lifestyle! Very different than mine and interesting!) good luck