I’ve never run a marathon, but I typically run 2 half marathon’s per year. At least in a half marathon, the OP is correct, it’s really hard to finish the race if you stumble too hard out of the gate. When you’re stuck at the back of the pack and can’t stretch your legs, find your comfortable pace, you get bogged down in frustration.
I imagine that is true of baseball as well, and some of the wisdom in the “consistency” phrase that gets thrown around. If a player is inconsistent, they are too reactive to the dictation around them. In the long distance run, it’s finding your place at the gate before the gun fires.
too far forward and you’re being stepped on, shoved and trampled by runners who are way faster than you, or sprinting and running out of gas… Too far back and you can’t get through the pack. Find the right spot and you can set your stride and maintain. Adjustments come along the way as you avoid obstacles, but it’s much easier to find your pace again when the pack isn’t dictating your speed.
Similarly I suppose, for a hitter. Adjustments are the name of the game, if a batter is too responsive, pitchers and catchers can manipulate and dictate how the at-bat, game, series and season goes. Consistently keeping within your game, making small adjustments, not big ones, and practicing patience can help the batter not give the pitcher too much sway.