It is an interesting question, but I think the root of it was a bunch of years of track intervals doing 400's, 800's and 1600's and knowing my pace to the second per lap. Because on the track, you don't see a continous number versus pace, and only see it twice per lap (or we could say 4 times per lap every 100m), you end up fine tuning pace to pereived exertion to different points in a workout.

25 years later when I started doing masters swim racing, my coaches were suprised by how well I managed my splits in say a 1500m or 800m swim race, but that just came from fine tuning that at the track. On the bike the way to do this is actually put your speedometer and power meter away and do short TT intervals on a repeatable circuit and look at your split times....its a bit impefect, but if you have a power meter and you can cover it up and see what power you rode on each interval and try to get it as steady as possible without looking at the numbers (but look at the numbers post workout), it actually does not take that long to get there, because after a while without seeing numbers, your brain and body get tuned to what you can sustain. If we could not do this our species would be roughly extinct, because we would never be able to self calculate the maximum sustainable speed for a given amount of energy stores. Amazingly humans are very very good at this and self select a pretty good pace, but you have to train that.

The other one is mapping out a 5km run course and pick random landmarks and (could be a house on the road, a tree, a traffic sign) and try to guess the time that you arrive at each, but don't look at your watch in between. If you do it enough, that same brown house, you will arrive at within seconds of "last outing at the same effort". You'll get good at it, but you have to tune your internal guage.

I also have a 1km interval section on a local road where I know my exact split numbers at four intermediate points (and they are not evenly split apart) at different perceived exertions between 10K race pace, 5km race pace, ironman pace, warmup pace. Eventually you get all that dialed in, but you have to come up with some fixed landmarks and use a basic watch.