I gave not enough details but too many thoughts.
Going back 2 years before I tore my meniscus, I was doing run/walk on a regular basis. During my run segment I was running faster than the 5k pace on my current plan and did not need to think about my pace, I just did it. If the run was 1.5 hours or longer it took a lot out of me for the rest of the day (lifeless blob). I know there were probably other variables contributing to this: inappropriate pacing & interval length (runs always felt difficult), and weather conditions are some.
I should have said I was running sporadically during the 18 months prior to February 2019 . The vast majority of these runs were less than 2 miles, with the exception of brief training for Dark Side 2018. In the past 18 months I did not run fast nor did I try. I know being 2 years older probably is a factor. I'll admit also that my leg strength is not what is was before my surgery, not just in the leg of the injured knee, but also the other leg. (so maybe I should not have said that I have "appropriate" muscular fitness).
So 2 years ago I could do a run interval at a faster pace than my current 5k interval pace and did not need to think about the pace. Now, I need to think about it and I have noticed that the segments during which I was not thinking about it my pace was slower than desired. When I'm telling my body to go faster I usually just focus on increasing cadence while maintaining form and breathing pattern and then I'm able to go faster.
When I say that yesterday's run was "challenging" what I mean is it was more exertion than I'm used to but not so much that it feels unsafe/unachievable. But afterwards I was able to carry on with my day and do other things. (challenging=uncomfortable exertion in that moment, yet achievable).
I think the GOTR race in December was probably a slightly different situation. I had a Rabbit and when I had to speed up to catch her it felt easy. I focused on my form and was surprised that I was able to briefly run at a pace that I would not have otherwise attempted. I guess I just need a rabbit?
Thanks!
I know this still sounds confusing. Two years ago I didn't think about going fast, I just did it. But maybe I exploited this and instead of being challenging, the workouts were more intense and more difficult, thus wiping me out for the day.
So what it sounds like you're describing to me is that it was easier back in the day to "just run" and not have to think about it as much. But that when you did this, the runs left you feeling more fatigued throughout the day.
These days with specific goals on pace, means you have to be more focused to actually hit pace goals. And if you don't focus during the run on those pace goals, then you find yourself not hitting the goal pace. But with appropriate pacing, you're not as sore the rest of the day.
So it would seem there are two different things at play:
1) You're likely less sore because the runs are more appropriate now. Rather than a laissez faire style where you just do whatever, you have something a bit more systematic. The systematic design has reduced the fatigue because the plan is more focused and balanced.
2) But with that more systematic design the runs during can be more mentally fatiguing since you no longer can zone out as the goal is to hit specific pace goals. So you need to focus mentally to remind yourself to go faster or hit a pace.
So #1 is clearly a good thing. We want the plan to be more focused and balanced. Having a medium level fatigue constantly rather than a super high and super low level of fatigue oscillating.
#2 is something that over time will fade. I know from personal experience that I couldn't blindly pace myself to save my life. After years of hitting pace goals, and learning what certain efforts felt like, I can now mostly go out the door, say what pace I want to hit, and then just do it. Like this past weekend with a pace goal of 6:57. I barely had to look at my watch at all because I "know" what that pace and effort feels like. So I now just go out and "do" it, which frees up my mind mentally. I obviously have to give more effort and tell myself to go faster, but now I can just lock on and hold it without much additional thought. So the need for mental effort removal will take time, sometimes weeks, months, years, decades, or never at all.
As for the rabbit, competition and group runs are a way of achieving cognitive dissonance. When you're running by yourself, it's all about you. Your pace. Your effort. Your thoughts. In a race, group run, or chasing someone else, it's not all about you anymore. Now the focus is partially shifted away from you to the other person(s). So that removal of feedback from your mind can sometimes be freeing because it's not as mentally fatiguing to think about the pace as much, because you aren't thinking about the pace. This can be good and bad. That feedback sometimes tells us warning signs when a pace is inappropriate. Leading to going out too fast because the group did, and then suffering at the end when you didn't heed warnings. But conversely it can also be beneficial, because maybe getting out of one's own mind allowing them to focus on other aspects allows them to run faster than they "think" they could.
Does that sound about right to what you're experiencing?
I would also be curious about LDL LLD. Maybe you're one of those people with a genetic mutation that keeps their LDL low 
(probably not, their LDL is usually around 10). 
