Olympic sprinter Gabby Thomas can routinely break 22 seconds in the 200 meters and her 400 meter PR is 49.x. Every January, she does a 1 Mile time trial. I would have guessed that she would be able to run it in close to 5:00 if not slightly under. Nope, her best time is only a 5:43. This is not a knock on Thomas and I applaud her for trying. My point is that there are specific physical attributes necessary to be successful in long distance running and most sprinters don't have it. If she actually trained specifically for the distance, yes she would improve but would never even sniff world class.
Though I've done many 200 intervals in training, only once did I race the distance in competition. I clocked a 28.6 as a high school Sophomore on a dirt track with tired legs. Figure that as a Senior, I was good for a 27-flat, maybe a high 26 at best. I would be a full 5 seconds behind Gabby Thomas and in a 200, that might as well be light years away. However, if we raced a Mile, my 9th grade self could take her. An 800 meter race in my prime would probably be a coin flip.
At the age of 17, I entered a local youth Invitational meet in the off-season. There was no gender separation so I got to run in the same heat as a top female sprinter in the 800 because our predicted times were similar. Her 400 PR was about 5 seconds faster than mine but was untested over 2 laps. I trailed most of the way but she faded down the stretch. I unleashed a monster kick and nipped her at the line by about a tenth of a second. My Dad said it looked like I ran the last 100 in 12 seconds. He was exaggerating but yes, it was fast. I didn't exchange words with her because we were both doubled over in pain but gained mutual respect. A couple guys teased me about never allowing myself to be beaten by a girl. LOL. Losing to a champion would be no disgrace.
I knew several 400/800 runners who ran cross-country to stay in shape for track season. Most were not very good. There was one guy at a rival school who placed at States with a low-48 in the 400. He was competitive from the 100 all the way up the Mile, with a 4:54 PR in the latter. However, only once did he even break 22 minutes in the 5K and didn't even make Varsity for his team. He told me that he did make a legitimate effort in cross-country but not the same passion as Track. It was pretty cool to beat him in a race even when he was out of his element.
A distance runner can never be a world class sprinter either but I wager that they'd be a lot more competitive in the 100 than a sprinter in a Marathon. Even with superior endurance, some degree of slow down is inevitable with longer distance. You've got to have pretty decent sprint speed to run a sub-5 pace for 26 miles. My guess is that world class marathoners can run the 100 in less than 11.5 seconds (about 83 percent of world record). Some of those sprint finishes in the 5,000 are awfully fast. I'm talking 52 seconds for 400 after nearly 3 miles. By comparison, 83 percent of a world record Marathon would be a sub-2:30. Even an 800/Miler would be hard pressed to do that. Quenton Cassidy is fictional.
Advice for sprinters who want to extend their distance:
Do it gradually. Don't make the jump from the 400 all the way up to 10K. Don't go by the book in terms of training paces either. It's okay to be a little faster than prescribed for your Mile time in interval sessions if your 400 PR is much stronger than your Mile. Also, it's okay to run your long runs a little slower than suggested if your 5K is weaker. It's all about the effort. Eventually, long runs will get faster when your times begin to even up. That said, once again, although there is room for improvement, your ceiling is likely lower at the longer distances.
