(CNN) Ryan Hall may hold the fastest marathon and half marathon times for a US athlete, but these days he admits that running doesn't feel like it used to.
"It just feels like you're wearing a weight vest the whole time," says Hall, which is hardly a surprise when you consider his physical transformation since retiring from the sport five years ago.
As a professional runner racking up 120 to 140 miles a week, Hall would weigh in at a lithe 130 pounds; today, he's 190 pounds and far more likely to be found lifting weights than grinding out 20-mile long runs.
And while he may not be able to get close to his half marathon record of 59:43, nor his best marathon time of 2:04:58, the 39-year-old Hall says that he feels all the better for it.
"My body was just done -- I couldn't even finish an easy, 30-minute run at that point," he tells CNN Sport as he reflects on his retirement from professional running in January 2016.
"I'm five foot 10, I was 127 pounds and my body was just totally depleted and in really bad shape, and I needed to get back to my body.
"Weightlifting became a new medium for me to express who I am, which is a guy who loves physical challenge. I love being active. I have to work out every day or else I'm grumpy and in a bad mood."
His current workout routine involves daily weightlifting sessions of 60 to 90 minutes, squeezed in around his coaching (he trains his wife Sara and 10 other professional athletes) and parenting commitments.
Among the benefits of lifting compared to his running schedule, Hall says, is an increase in his energy levels, which are "10 times better" than when he was running professionally and would rely on taking two-hour afternoon naps each day.
He says that his testosterone levels were always "clinically low" as a runner, with natural remedies making no difference.
"It's just a really hard profession on my body," Hall adds. "Whereas weightlifting is totally the opposite of that: it's anabolic in nature, so I'm building muscle and building strength.
"It is a great way for me to give back to my body after it had given so much to me over a 20-year period. Now I did finally give back to it and I feel amazing now, my testosterone is back into the normal level, my energy's better, my motivation's better.
"I'm a better dad, a better coach and a better person as a result of it."
Amid his transition from professional running to coaching, Hall's desire to push the physical limits of his body hasn't diminished.
Rather than testing himself over the 26.2 miles of a marathon, he will take on unique challenges combining strength and endurance.
Last month, for example, he took on a wood chop/water carry challenge. That involved splitting a cord of wood and running 6.3 miles to the bottom of the Grand Canyon, then 6.3 miles and 5,000 feet of elevation back up while carrying a 62-pound water jug in each hand.
"It kind of reminded me of my pro running days being on the starting line of a marathon and trying to run 4:45 per mile for a marathon distance and just being like: How am I going to do this?" Hall says of the feat.
"Even though you're trained, you're prepared, when you get in that moment it's a scary place."
He's now mulling his next challenge. One goal is to try and complete a 500-pound deadlift and mile run in under five minutes (he got 5:28 for his first attempt), and another is something that involves "covering a good amount of distance with a heavy weight."
However, he's hasn't entirely forgotten his love for racing marathons. Coaching his wife Sara, who has run the second-fastest marathon time ever by an American woman, serves as a reminder of that.
"When I'm at Sara's races and she's super fit and she's warming up for marathons ... I just remember: Oh, how good it feels to be ripping it on the roads, feeling like a million bucks floating through it. There's no sensation like that," says Hall.
He continues: "I can think back to a whole bunch of moments when it was just me out in the woods on single-track trail, no one else around, no other sounds besides the wind blowing through the trees and having that same exact sensation: being in full flight, feeling like I'm floating along."
It's the sheer thrill of running that Hall tries to pass on to the runners he mentors, including his four daughters.
"I try to remind my girls: Hey, it's not about the performance, it's not about what place you are. Can you just fall in love with just the sensation of your body being in flight?" says Hall.
"Because if you can fall in love with that sensation, you're going to get everything out of running that you want to get out of running.
"That's where the good stuff is at. It's not hitting a PR [personal record], it's not finishing a certain place or going to the Olympics or any of this stuff. It's in the beauty of the sensation of your body in full flight."
On top of the 11 athletes he trains personally, Hall has also started Run Free Training, an online training platform of around 150 athletes.
He is, he admits, "a much better coach than I am an athlete." While he was "super rigid" with his schedule during his running career, he's learned to appreciate the value of constantly making adjustments to an athlete's training routine.
"I like to see myself as almost a chef," says Hall.
"When I'm writing the training, I have a 'recipe,' right? But based on what I'm seeing, based on the feedback I'm getting, it's like I'm tasting it as I'm cooking and then I'm changing up the flavors, throwing this and that in there, taking this, that out, making it just right."
He has a similar approach to his own training in the weight room, making tweaks to his workouts to challenge his body in new ways.
"I've got to be throwing new stimulus at my body that it hasn't seen before, so I'm constantly finding new ways to do that," says Hall.
"I've studied a lot of training, I get inspiration from other people's training, but then I also get creative and I do stuff that sounds fun ... I try to be really intuitive with my training."
Hall has also tweaked his diet over the past few years. As he did when he was a runner, he still eats a meal or a snack every three hours, but these days he's also upped his calorie intake.
"I'm eating a ton of protein and I'm still eating a ton of carbs," he says. "Basically, I'm eating a lot more calories than I used to eat ... I'm trying to eat 4,500 to 5000 calories a day, and if I'm not, no matter what I'm doing in the weight room, I'm not seeing strength gains."
Away from their life in Flagstaff, Arizona, the Halls have supported development projects in Africa, which has included fundraising to bring clean water to 90,000 people in Zambia and for a new health clinic to be built in Kenya.
They have also founded the Hall Steps Foundation, a non-profit organization that protects vulnerable women and children living in extreme poverty in Ethiopia.
In 2015, the couple adopted their four daughters from Ethiopia; Hall says the family plans to set up an initiative there in the future.
"We want to do our part to help," he adds. "Our kids are so amazing; they blessed us way more than we blessed them."
Until the family can return to Ethiopia, Hall's focus remains on coaching and imparting the lessons he learned throughout his running career; it makes the injuries and setbacks all the more valuable.
"I'm passionate to share these things that I've learned the hard way," he says. "I love it because it redeems the failures that I've made along the way."