I’m 72 and have been a consistent runner-cyclist since my sophomore year in high school. While I enjoy long runs, I also mix in Special High Intensity Training, which are not fun - (thus my friends and I refer to them as S.H.I.T. sessions).
There are some guys on this forum who are my age and older, who are really fast. I’m not one of them.
As far as running goes, I’m not a sprinter but I was fast.
I could run a mid-marathon in just a little over an hour and not be tired.
Basically, my running style was that I was fast enough to catch most endurance runners in speed and I could catch most sprinters in time.
I find HIT running to be useful in building endurance. HIT running is hard, no question. However, the level of endurance you build if you tolerate the training is more than you could ever obtain by running miles.
Having said that, the whole point of running should be to run out of steam. You may have a low tolerance for sprinting until you run out of steam, but you can still push yourself to that point.