And indeed there is always more fitness that I need.
Outside of a goal race, you shouldn't suffer when you run, especially 'a lot'. You are tearing down muscles and training energy systems that are inefficient.
Slow down. Walk. Fragment runs into multiple runs. Run doubles. You'll get faster, faster. Add volume at your new, sane pace and new, sane volume up to 40-60 miles/week.
Once you've achieved the slow end of 'adult fast', say sub-20m/40m/90m for 5k/10k/half, you can revisit suffering during training. You still probably shouldn't.
Of course if you want/like to suffer or can only run 4 times a week at peak heat during a Georgia summer then all of this is out the window.
I'm not much into sport, but I had the impression that training was above the goal.
Like, you try to get into a situation that is harder than reality, so when you finally do the actual thing, it will be easier.
Often the strategy is to adopt both, until you gradually converge on some target (pace over distance). Where that target is is a different issue.
Plus, fresh of the couch everything hurts. Some fit person's lungs might be burning, their legs might feel heavy, but the not-fit person has three different cramps, blisters, achy feet, shoulder pain, toe pain, neck pain, lung pain... that gets better fairly quickly with training.
I feel the whole "it never hurts less you just get faster" viewpoint is demoralizing & misleading, which doesn't help when people are struggling to stay fit.