Yeah, that's my general understanding of LT pace, but that pace is still pretty tough for me to imagine for a 4 mile LT run. I suppose that's mainly because I never do LT runs on the track and I've always got those things that increase perceived effort and whatnot.
Here's the way I think of it (which is exactly what you just described, but in different words). LT is really measured by effort, and there is a certain level of effort that will put you right at the LT line for ~20-30 minutes or so depending on how well-trained you are. At some point, other factors will come into play for fatigue, and running that same pace will put you at a higher effort which will put you over the LT. So like you said, if you want to go for a longer period of time, I think it's best to start at a slower pace, but eventually that slightly slower pace will put you right at the LT without going over during that extended time compared to the shorter LT run.
So that's my reasoning for doing moderate paced long runs and why they are so beneficial for true distance runners. If you average ~40 seconds slower than LT pace for a long run, let's say an 18 miler. Running ~40 seconds slower than LT pace will not be very difficult for quite a while. Since you're doing a long run though, that effort will eventually creep up to the LT and you will spend a good amount of time right around that effort for the seconds half of the run. Then you're getting all the benefits of a long run plus the benefits of working the LT.
So why do I pick 40 seconds slower than LT pace? Well, that's just purely anecdotal evidence. I'd hear about some different great runners doing several long runs at x:xx pace when they have certain PRs, and I compared that long run pace to what their LT pace would be, and it was pretty common to be around 40 seconds slower. It seemed to work really well for me this cycle.
I might be way off base, but anyways, that's how my mind comprehends this topic. Thanks for the congrats!