Great detail in that question, thanks :)
Let me start by saying that you should always get the advice of your physician before making changes to your exercises routine or changing your nutrition.
It's true that lower levels of protein can affect you negatively. The amino acids that comprise protein are essential for the transportation of oxygen throughout your blood stream. Lower levels of protein can reduce the amount of oxygen available to your system, which may exhibit itself as feeling of tiredness or fatigue. Having lower protein intake could also reduce the amount of muscle your store, which has a knock-on effect of reducing strength and endurance. This means you have to work harder to achieve the same result, hence feeling even more tired or out of breath.
You ask if you might be just getting too old? NEVER! He he... With the right fuel, correct nutrients (maybe a daily vitamin, iron, omega 3) and training you can go on till you drop. I have clients in their 70's who exercise five times a week. One is 78 and runs 10km twice a week!
I'd suggest talking to a professional about your diet, trying to find the amount of protein, carbs and fats that are going to work for you, given the challenges you are taking on.
I offer all of my services online as a naturist PT, so if I can help any further, check out my website at www.nkdfit.com.
Thats one area i have done plenty of research on, being a vegitarian myself. While protein is the building material for muscle, it serves as poor energy to operate them. Its carbs they burn tor energy. But our diet in general is a very "Dirty carb" diet, causing very inefficent energy delivery. Even though Im a very fit man for my age 52, I can hike many miles with little fatigue, when i hike fast uphill i run out of steam pretty fast. I dont like that and want to do something about it. And there happens to be something I can do it appears. Dietwise. It appears a raw vegan diet will deliver very clean energy delivery. So clean and so good it will allow massive increase in endurance capacity and way more energy per calory. But Ive heard two sides to it. It doesnt always work. And Im sure I know why. My research in agriculture ties in. Besides the fresh food avalible is mostly tasteless junk. Pretty boring diet. The amount and quality of energy avalible in fresh food from plants is highly variable, depending on soil minerals, balance between them and amount avalible, as well as the soil biological and fungi health. I intend to experiment on it myself, grow my own diet with the best growing environment i can provide. Mineral feed to the plants in the way they like it, and see what it will do for me. I will report what im doing in that realm on the gardening group since that is like 99% of what there is to it. id like to see how interesting and flaverful i can get it, and what results i get from eating it long term. Because diet is a long term effect thing. Cant expect instant results, but over months i should see some change. I read of a young athlete running 100 miles per day several days in a row on 500 calories per day. Thats extreme performance. I cant hope to get close to that at my age, but Id like to see what i could do. Where i live i can trim a trail through the woods up the mountain to meet the end of a logging access road, it would be more or less steep uphill for a couple thousand ft, then relativly level 3 miles out to the main road. Good test run, 6 miles round trip if i go all the way. I hike it naked occasionally. (always hike naked unless conditions or company makes it unadvisable) Usually round trip on a different return route. But it needs a lot of trimming to get through easily. A lot of brush and briars came in after the logging crew thinned the trees.