In the last few weeks, Norway's top triathlete has run some pretty impressive 5km times: 13:51 and then a few days later 13:48. A couple of other Norwegian triathletes have run only slightly slower. He thinks he could run 13:25 or so if he concentrated on running.
https://www.kondis.no/triatlontrio-paa-topp-paa-5-km-i-perseloepet.6314904-127710.html
What I find fascinating about this is that these triathletes do not look, to me at least like, typical runners. They are very muscular, see for example the picture in the following article.
They train hard: 570km cycling, 160km running, plus strength training.
What I conclude from this is something that I think should offer optimism to a lot of runners who are put off by what are often asserted as limitations of weight and of raw speed. I do not know for sure, but I seriously doubt that these triathletes ever had remarkable speed in their younger speeds. I suspect that most of that speed is coming from their muscular strength, i.e. I suspect they have become faster over the years.
Cycling strengthens the leg muscles much more than running ever can do, no matter how many hills one runs. On these boards, one often finds assertions that muscle is not necessarily helpful due to the added weight. I think this is wrong. One needs speed to run quick 5km times, and one way to gain more speed is to gain strength. As long as those muscles are trained to be able used in a way useful for running (i.e. aerobically as well), the benefits will outweigh the extra weight.
I have seen another example recently of a local runner in his mid 30s who does a lot of cycling (running and cycling on alternate days mostly), with long cycle rides at a good pace combined with quality running workouts and occasional easy runs. He has improving and improving, and latest runs about 15:10 for the 5km.
My conclusion: contrary to a lot of assertions made on these boards, a) they may be something to a model of quality cycling plus quality running for improving running b) one is not necessarily limited by raw speed, one may get faster even though one is training for middle to long distances if one trains in an appropriate way.
Thoughts welcome!