Yesterday during a run, I mixed in a few bursts of fast running. It then seemed like after each burst, the corresponding easy running portion afterwards somehow became (or at least felt) easier.
I will also mention that this isn’t the first time I’ve recognized this phenomenon.
I don’t think it’s a matter of faster running “warming up” the muscles, as I had already been running for 15 minutes or so.
Here’s an analogy I’ve been thinking about. If you always lift say 100 pounds, then maybe you can build your endurance for that weight, but it may never truly become “easy”. But if you strengthen and learn to lift 200 pounds, and then go back to 100 pounds, it should now be easier relative to your increased strength level.
I wonder if the same applies to running: faster running builds your system or gives you more metabolic “headroom”, such that it makes slower running easier.