My first time racing a go-kart was a blast. I was "OK"; quick enough for a new-comer, I could pick out the line, and I had enough sense of the others around me to avoid most collisions.
But once we pulled back in to the pits my forearms were on fire. My shoulders arms and shoulders were sore. I had exhausted my abdominal muscles. I was breathing fast. My heart rate was racing almost as fast as I thought I had been going for the last 8 minutes. I was not ready for this at all. I had no idea this is what it took to wrestle something around a track at a decent pace.
If you were to ask a stranger if a NASCAR drive were an athlete, the answer would almost certainly be "No". They would be wrong. On a typical race day, these drivers endure anywhere from 3 to 6 hours in the car. The car is not a 72 degree office either; it's easily 120 degrees and can peak at temperatures above 150 degrees in the summer. There is no air conditioning. A driver's core body temperature can surpass 101 degrees. They must endure this better than their competition.
In Formula 1, Lewis Hamilton is the top dog and has been for most of the 2010s. His cardio game is on point, running 10s of miles every week to stay in shape. Daniel Ricciardo considers a 2.5 hour bike ride "solid". This level of fitness is usually comparable to marathon runners.
On top of rigorous endurance work, the best drivers also train agility drills, strength workouts, and core strength. The latest rules change for Formula 1 sees cars cornering at up to 6.5Gs of lateral load in the highest speed corners, G forces usually reserved for fighter pilots. If you've ever seen the video footage from an Air Force G simulator, you have a very good idea of what this does to humans. These forces mean that muscular endurance needs to be top-notch. Neck and core muscles must be resilient. Under 1G, your head might weigh 10lbs. Add 2lbs of helmet to that to make 12. Now multiply that by 6. At 6Gs, your head with a helmet will feel as if it weighs 60-70 pounds. Do that for 90 minutes.
Mental agility is paramount. Under those kinds of stresses, it becomes difficult to make clear, rational decisions. Circuit of the Americas has 20 turns. Each one of those turns requires multiple mental calculations that must take place to execute a fast lap. Hitting your braking point, bleeding off the brakes and blending in wheel, nailing the apex and blending on throttle, and tracking out or setting up the next corner all require immense focus and execution to do perfectly.
The obvious benefit is that being lighter as a driver is "free" weight loss for any vehicle driven. Not only does the work put in build up the cardio-respiratory and muscular endurance needed to gain the extra edge, it also reduces the amount of influence the driver's weight has on the car's total weight, CG, and corner balance. The effects of this grows as the car itself gets lighter.
Often times when we look for that next level of performance, we overlook the importance of a fitness program to build speed. We tweak and tune and turn wrenches on the car, or we might hit the books to gain some insight on how to approach a particular type of corner or a certain type of car control. The numbers might be different for us mortals, but the difference between the fast guys and the aliens could be as simple as who hit the gym that morning.