The 'known' slow twitch athletes are elite level athletes in peak condition - they would beat almost any human on the planet in almost any running event.
Put them in an elite level sprint field and the differences would start to show. Putting on my speculation glasses - I'd guess an elite marathoner could beat High School level runners at 200M. By the time you got to state championships I think the fast-twitch sprinters would dominate and by NCAA or national level competitions the marathoners would be left for dead.
Conversely - if you took a world class sprinter and tried to turn him into a marathoner - you could probably make a pretty fast marathoner. These are great athletes, with training discipline, determination, etc. You could get them to sub-3 hours, maybe approaching 2:30 which is a fast marathon by almost any standard. The only standard it is NOT fast against is elite level marathon.
Talent, training and hard work will take you a very long ways - but to get to the absolute peak you need some specific physiology.