No, if you're a professional athlete this should not be all that difficult. Boring but not difficult.
Look at the average baseball player. So many of them are plump or overweight. I'd argue that soccer players running 10-15 miles every game have a bit more to deal with.
In general, the public has a poor understanding of being a professional athlete. Typically, when I try to explain it to others, I usually just say "it's a job like any other" - the same as staying on top of your game in the music industry, running a startup, etc.
It's easy for us to dismiss the effort professional athletes put out when you only see them on TV a few times. However, it's somehow different when we see Steve Jobs or Tim Cook on stage a couple times each year. We recognize all of the hard work Steve/Tim likely put in behind the scenes.
For some reason, a lot of people don't recognize that same level of effort for professional athletes. They just assume they show up for the game and perform at a high level.
For every hour on the field, there's probably two hours of study (mental preparation) and two hours of treatment and/or rehab (physical maintenance). Pair that with a rigid schedule every day, fatigue is a real concern.
We all know the overnight success myth. The myth exists for professional athletes as well. Athletes that make it to the highest level didn't simply wake up one morning and have all the tools to succeed. Hours and hours and hours of training, both mental and physical, had to occur. It's like anything else - you don't simply get good. It takes practice.
It also takes commitment, dedication, and perseverance to succeed at a professional level - just like everything else in this world.
A first baseman, sure. They only need to be able catch well and sometimes throw across the infield. Find me an out-of-shape center fielder. Miguel Cabrera, for example, is a bit bigger. He's slow getting around the bases. But his talent, other than being a first baseman, is hitting the ball hard enough that he can casually stroll around the bases if he wants.
But humans were meant to run. That's why we stand upright, that's why we sweat. Running long distances on a flat grass field, to a trained athlete, is going to be less fatiguing than standing still in the sun waiting for something to happen. Pitchers throw 100+ pitches at 90+ MPH and for the most part, are dead accurate. Even if the ball is spiraling through the air to confuse the batter. And the players, other than the starting pitcher, are out there almost every single day, and sometimes play two games per day. Baseball has a ridiculously packed schedule.
So I would disagree that soccer players have it rougher. Running is easy.
Baseball pitchers regularly play well into their 40's. Jamie Moyer was still pitching a month or two shy of turning 50. For soccer players, still playing at 40 is nearly unheard of. Ryan Giggs did it, but that's about it, and it was mostly sentimentality that kept him playing that last year or so. So I would disagree that baseball is that much more taxing than soccer.
"Baseball pitchers regularly play well into their 40's"
Unless you're someone like Mark Prior who had their body give out when their career was just getting started. Why are you only highlighting the outliers, the genetic freaks (amongst genetic freaks no less)?
The thing with baseball pitchers is that there's an enormous selective pressure on their bodies (primarily shoulder & elbow) being able to withstand the brutal toll throwing all those pitches causes. The absolute best pitchers (talent wise) might never even make it to the MLB because their arm falls apart before they get a chance. There are pitching motions which hurt my arm just doing them in slow motion, now imagine heaving a baseball 90+ mph repeatedly for your entire life.
This is a far different selective pressure than what e.g. soccer players would have where quickness & speed on their feet is what matters.
The problem with these discussions is that people are using words like "taxing" to only narrowly mean what they want it to mean.
There are lots of Major League pitchers in their 40's. There might be no Premier League outfield players in their 40's. Lots of players in both sports get injured and end their careers in their 20's.
No doubt that a starting pitcher's pitching arm takes a lot of abuse, but they can have much longer careers than almost any other athlete.
That's not my point, what I was saying is that the two have completely different curves due to relying on completely different physical attributes.
A soccer player (or NBA player or many NFL positions, etc) is going to deteriorate steadily as they age after hitting some peak year. For some sports there'll be a slight bubble after that ast the intersection of athleticism decreasing and increasing wisdom but not for all. Sure, someone might endure a catastrophic injury and some players are made of glass (e.g. Greg Oden in the NBA) but generally players will wash out either when their athleticism is not high enough for their level or it deteriorates below the level necessary.
A baseball pitcher's curve is different. Their arm/shoulder will either be able to withstand the long term abuse or it won't and if it's the latter they'll wash out pretty early on (likely long before they ever see an MLB field). Once they clear that hurdle the actual deterioration rate is much slower in the general case. It also helps that pitching is a position where accumulated wisdom can really help, a lot like the quarterback in american football and that can help balance out declining athleticism as well
People don't realize just how much pitching taxes the human body. There's a reason why most pitchers have several joint surgeries over the course of their careers.
I agree that one baseball game is less stress on an athlete's body than one soccer game. But soccer players play about 30-40 games a season; major league baseball players play 162 games a season.
The overweight players are overweight for the same reason that (American) football linemen are: it's useful for their skill.
One of the reasons that Prince Fielder can hit as many home runs as he does is that he has a lot of mass; it's also one reason that steroids were effective. Not (just) for strength, but the more mass you have to swing into the ball, the farther it will go.
I won't pretend to appreciate the challenges of being a professional baseball player, but I can imagine that the constant travel and resulting disorientation is a contributing factor to fatigue. This article discusses the challenges.
Look at the average baseball player. So many of them are plump or overweight. I'd argue that soccer players running 10-15 miles every game have a bit more to deal with.
baseball needs some yoga and proper nutrition.