Michael Phelps isn't the young man he used to be. The most decorated Olympian of all time is now 31 years old, and competing in his fifth Olympic Games since his debut in 2000. The American has put a ton of meters on that 6'4 frame. Nobody has participated in more Olympic events.

Now Phelps is yet again dominating in Rio, even though most of his peers are years younger than him. On Thursday, he'll swim in the 100-meter butterfly heats in the afternoon, then get a double dose at night with the 200-meter individual medley final and 100-meter butterfly semifinals. He'll have fewer than 40 minutes between the night races to cool down and get back out there.

The stacked schedule is a testament not only to Phelps' extraordinary physical gifts, but also his remarkable commitment to an intense, demanding training regimen.

Phelps has been been training and competing in the pool most of his life. He made his Olympic debut at age 15 and has spent the past 16 years proving himself as the greatest swimmer ever. At the 2008 Olympics in Beijing, his training and eating routines became the stuff of legend, when he revealed details of his hardcore workouts and a calorie intake of up to 12,000 per day. While he's since backed off that calorie figure, calling it a "myth," those Games not only earned Phelps a record eight gold medals, but helped show how he does such amazing things.

Things are a bit different now, not only because Phelps is older. He's also wiser and more appreciative of his experience now, less stressed by the pressure of his unique job. At his age, he's needed to change things up a bit, but as you can see in Rio, Phelps' altered routines have clearly worked.

Intense endurance training

Phelps' training demands are well known at this point. In 2008, a reporter from Men's Journal underwent a version of Phelps' routine with the help of his coach, Bob Bowman. We got a glimpse into an intense workout designed to improve Phelps' endurance, which is critical on days like Thursday when he's racing in multiple events.

Here's a breakdown of that training routine:

Warm up with jackknife crunches (three sets of 20), push-ups (three sets of 25 to 35) and bodyweight squats (three sets of 25)

Begin in the pool with 50-meter drills, including using a kickboard to isolate upper or lower body on different laps

At moderate intensity, swim distances of 50m, 100m, 150m and 200m, with 30 seconds of rest between each interval

At high intensity, swim distances of 200m, 150m, 100m and 50m, with 30 seconds of rest between each interval

To finish, do 12 laps "with a pool buoy between your legs, breathing only four times on each lap," then do five more laps alternating between sprint and relaxed paces to cool down

That was the 2008 workout, so it's possible Phelps has made some tweaks here and there, but that general structure has served him well throughout the years. He's also talked about the importance of vertical kicking and underwater kicking drills, which he believes improve his ability to maintain speed over distances. Training outside the pool, such as weightlifting, is common, too.



However, there is one real difference between 2008 and now: Recovery time. Phelps' training sessions don't last quite as long as they use to -- more like two to four hours per day instead of five to seven -- because he's taking more time to rest and recover. Learning how to manage your body is a crucial aspect to getting older, and while Phelps is as committed to his training as ever, that means working smart, too.

No more long-distance races

One key choice that Phelps made to ensure he wouldn't get overwhelmed in Rio was to avoid the long-distance races. Phelps isn't competing in the 400-meter individual medley this year largely because the longer distance takes a bigger toll on the body. The American instead wanted to focus on the shorter sprint races at 100 and 200 meters, where his power is an undeniable advantage and his age is less likely to catch up to him.

That's pivotal for someone who competes in as many events as Phelps. He's good enough to swim in almost any event, so at his age, he needed to decide which ones to prioritize. Phelps won gold in the 400m IM in Beijing, but finished fourth in London, so it was a natural choice as something to cut from his schedule. In doing so, Phelps is better positioned to thrive in the events where he is suiting up.

And it's easy to see why given Phelps' schedule. He's been competing every day in Rio, and on Thursday, he's going through a grind. In the afternoon, he'll be swimming in the 100m butterfly heat at 1:31 p.m. ET, then he'll be in the men's 200m IM final at 10:01 p.m. and the 100m butterfly semis at 10:42 p.m.

That's a stacked schedule, and many of the athletes Phelps is up against haven't had the same kind of workload, either in Rio or over the past 15 years. There are a lot of things that make Phelps superhuman, and his endurance is high up on the list.

A diet not for everyone

Athletes at the Olympics can have pretty wild diets to sustain their training and competing regimens. Phelps is no different, although he's changed his eating habits over the years to accommodate his age. Much was made of his 12,000-calorie comments back in the day, and while that exact figure might've been an exaggeration, Phelps eats a ridiculous amount of food to fuel himself.

We're talking loads of proteins and carbohydrates, along with a healthy dose of fruits and vegetables. His breakfasts are typically massive, covering just about every category of breakfast food you can imagine, and he's long loaded up on pasta. While he had indicated in the past that he was trying to move away from such a carb-heavy diet, Phelps told reporters he ate a pound of spaghetti after the 4x100m relay Sunday night in Rio.

He also stopped drinking alcohol in 2014, but not directly for training. That year, Phelps was charged with driving under the influence, which led to a suspension and a trip to rehab. He hasn't drank since.

"Before I even went to court, I said to myself that I'm not going to drink until after Rio -- if I ever drink again," he said before the start of the Games. "That was a decision I made for myself. I'm being honest with myself. Going into 2008 and 2012, I didn't do that. I didn't say I was going to take a year off from drinking and not have a drink."

The lack of alcohol is now part of Phelps' improved diet. You need a ton of calories to burn to push your body the way Phelps does, though, so unsurprisingly, that means he's stuffing his face regularly.

Embrace the fun

This might be the biggest change between Phelps from four years ago. After the 2012 Olympics in London, where Phelps won four more gold medals, he retired from swimming. He said he no longer enjoyed the sport and wanted to move on with his life. Phelps had seemed to be going through the paces, and while that was still good enough to win, it wasn't enough for him.

Phelps returned to the pool in 2014, but the comeback got sidetracked later in the year when he was arrested for DUI in Maryland. He received a six-month suspension from USA Swimming and, once again, was no longer competing. But somewhere between 2012 and now, through all those problems, Phelps rekindled his joy in the pool.

"I’m having fun again," Phelps said recently in Rio. "I’m enjoying what I’m doing again. I think I’m at the point that, whatever’s left, I’ll be able to turn the page and say I was able to finish the way I wanted to. And to me, that’s all that matters."

Phelps isn't just older, he's also happier and wiser. He'll still stare daggers at Chad le Clos when the South African rival is warming up in front of him, but now he's laughing on the podium during the national anthem because his buddies made an Orioles joke from the crowd. After winning the 200m butterfly gold, beating le Clos in the process, he cried tears of joy during the Star-Spangled Banner.

"I think the biggest change is just giving me a clearer head in the pool," Phelps said. "It’s obviously given me a much clearer head outside the pool in my family life, in my personal life. I’ve been able to enjoy life."

It all leads up to this

When you talk about all of this, from Phelps' training to his diet to his choice of races, the American came into Rio with a plan. He understood he could no longer push his body quite as hard, and adjusted various aspects of his preparation accordingly. He remembered how little he enjoyed his time in London, and made sure he entered Rio with a brighter, more positive attitude. On a day like Thursday, when he's racing three separate times -- including two races in less than hour -- you get to really see how that all adds up to make Phelps unlike anyone else.