I am 40 years old and after years as a "hobby jogger," just got serious about running this year. I ran a 3:37 marathon in October. I've read a lot about speedwork and am trying to begin incorporating it. I'm doing 8x400 sprints, then a 5K cooldown.
My 400 time has hovered right around the 80-second mark. My question is whether I should expect it to get better, or is this just a marker of my natural speed? And, if that's the case and I don't improve the 400 time itself, will doing this kind of speedwork still help my 5K and marathon times?
Thanks!