In my case, practice, knowing the course, and running close to my easy pace all help. I have a pretty good sense of what different paces feel like, effort wise.

But I also use a GPS watch and Stryd pod to monitor my pace in real-time and keep track of my 1 mile and 5K splits.

Generally though, runners tend to go out way too fast in a race because they are very excited and should have gone through a taper, so it feels easy while running too fast at the start. Because it's not a race for me, I can avoid that excitement. Then toward the end, they slow down because they went out too fast. Since the entire race is at a relaxed effort for me, I can avoid that too.

I also probably haven't spent the last 10 days tapering, so I don't have to really adjust my feel.

That said: a few years back I ended up pacing the 4:30 marathon group which was almost 3 minutes/mile below my race pace and I was constantly having to check myself. My preference when pacing is to be close to my easy pace for the distance, not to fast, not too slow.

Thousands and thousands of miles of practise. Elite pacers can hit 400 m splits with sub-second accuracy by feel alone.

At an elite event there are LEDs along the inside of the track that are set to pace at world record pace or what ever pace the meet chooses. The rabbit can pace off the LEDs, or at some interval to the LEDs. Recently there may be multiple paces set by the LEDs.