Not to mention there's always a large clock on the vehicle in front of them throughout the race? 😉
In Chicago 2002, there was a breakaway runner. The lead vehicles were out of his view in the late miles until KK took off, as I recall around the 24 mile mark. It's true that he ran with pacers who were surely in charge of hitting specific splits, though.
Cesar, in that 5K, they don't even have accurate mile markers. They have someone calling splits at about 1 mile and I know that one is close. They have someone calling splits near 2 miles also but I know the 2 mile mark is about 10-15 seconds before the person calling splits (pulling double duty, directing traffic at a turn also). So I heard splits along the way but they were, at best, approximations. I knew I was roughly where I wanted to be but I honestly thought in that race I was going to be flirting with 17 flat, hoping for high 16:50s. When I saw 16:40s and managed to cross in 16:47, that was a thrilling surprise.