In the middle of last year, I managed to take 10 seconds off my 400m time* without running a single effort further than 50 meters.
*Now to be clear I went from 65s to 55s. Hardly preparing for Tokyo.
Conditioning lives on a continuum.
From continuous work down one end (think marathons), all the way up to max effort and explosive training at the other (sprinting, jumping, lifting). The beautiful thing is there exists a magical trickle-down effect when it comes to getting fitter. We rave about intervals all the time but this flow on effect is by far the coolest benefit.
Your strength training will tend to improve your 10 to 40-metre sprint time, doing your 10 to 40m sprints tends to improve your 100m and 400m speed, as well as improving your repeat sprint and intervals capacity. In turn, consistently doing intervals will help improve your beep test and Yo-Yo score (both tests of aerobic and anaerobic capacity respectively) while also having a decent amount of transfer to your longer distance and marathon type fitness.
So while everything tends to flow down the continuum, the transfer don't necessarily always flow back up as readily. Your marathon training is not going to help (necessarily) with your repeat sprints or your 400-meter time*.
*To be clear this is not a black and white thing, there is some level of upward transfer, but it is nowhere near as effective as the trickle down that intervals provide.
In fact, sometimes marathon and moderate intensity continuous training can even interfere with high speed or power development (especially in the highly trained team sport and power athlete). For example, doing lots of steady endurance work can directly interfere with your speed and strength up the other end of the continuum.
This comes back partly to the AMPK mTOR relationship and partly due to muscle fibre and neural adaptations to the high volume, low-intensity training.