This is entirely my experience so give it whatever weight you’d like…
I was a fairly good collegiate distance runner but started to care more about the idea of becoming lean and light, rather than the actual performance. Ironically, it should have been the reverse, realizing that focus on performance would lead to an “ideal” body composition rather than the opposite. Anyways, I really f*cked up my eating patterns to the point where I was pretty much doing all of my training underfueld by substantial margins and also training a lot (100+ mpw). Now, 2 years after college, I’m still recovering from what should have been a quick injury thanks to imbalanced hormones and nutrient absorption and also tend to suffer from chronic anemia. I still train a lot but now it’s on the bike because my leg isn’t ready to run much yet. Eating has gotten better, but it’s very much a work in progress. So my first and main piece of advice is focus on your performance and let the body comp figure itself out. No one ever finished an Ironman and asked for your body comp stats, they ask about time, pace, watts (i.e performance based metrics).
As for things I’ve noticed when training while fueling vs not fueling, when I fuel during long efforts like centuries, I will recover substantially better and quicker than if I don’t. Like you, I’ve done long efforts like this with minmal fuel but they tend to lead to feeling like rubbish the next couple of days when I don’t fuel. You can also go harder more often (duh) when you eat because, ya know, energy. Lastly, I’ve found I’m substantially less irritable when I eat on the bike. When I’m chronically underfueld, I often get prissy and annoyed at even the slightest of things during a longer ride whereas fueling tends to keep me much more calm and relaxed.
Hopefully some of that helps!