when i first took my niece to the track she was in kindergarden. she knew she could beat me with ease. we did it first her way. the race was 1 full lap. i let her take the lead. in less than 50 yards she was fading hard. since she went all out. i said wait wait i give up you win. and we walked back to the starting line. then i said lets try it one more time. but this time you have to stay behind me until i say now. she played along. and we went out much slower. and she did not fade. but made it all the way around the track. and on the final straightaway with about 40 yards to go i said- go for it now. beat me now. and she "did".
when a person who has had the goal of doing a 10k , and has put in the workouts for the 10k, does the 10 k, they likely take the first 2 miles out way differently than a person who has has never done it, and never trained any distance. and maybe the low mileage cross training mile guy knows vaguely to take it out a bit easy. and he's got an all out 5;00 mile. so when he takes it out and averages a 5;40 mile one he's still feeling pretty good. and if he even looks at his split he thinks piece of cake i've got this made,,, etc etc... but if he had paced behind some steady ready old guy at 6;20 you know that last mile and 1/2 would be a whole different game..... somewhere between mile one and mile 5 the unprepared first timer is going to learn that pacing might matter. and if someone takes his splits for him and shows them to him after the race, etc etc.
but the average fun run 10k might have mile marks. and nothing else. and the average first timer is going to go by feel based on totally wrong experience. you give them a rabbit and what they can do is way faster than what they will going by "feel". but near all the unprepared novice milers are going to pace like total crap. then its just a matter of how hard they fade. everyone whos done a 10k knows there is some advantage to being familiar with paces related to your actual 10 k abilities. and when you arent geting any pace feedback until the first mile is done. unless you know the course and make sure you know for example where 50 meters and 100 meters and 200 meters and 400 meters are. a novice going to go out way to fast. 5 min miler might break 5 min pace the first 400 and not feel a thing. for a total novice its not the same game as a 50 plus master who knows his zones and makes sure he doesnt go sub 6;10 any of the 10k, its not even just the slow twitch fast twitch thing. its the pace like a dumbazz thing.