On the mend from a bronchial infection that plagued his last years of racing in Europe and the American domestic circuit, the 2013 Vuelta a España champion says his ability to train longer and harder, and the increased power levels that resulted, convinced him to throw his hat in the ring once more.
Horner will race under the Team Illuminate banner, the same program he raced with in 2015 when the team raced as Airgas-Safeway.
The breakthrough came for Horner when after more than a year of trying to diagnose the problem, doctor's finally identified his ongoing lung issues as a byproduct of acid reflux that allowed fluids to flow into his lungs and cause infection.
"The doctor originally thought it was a PFO in the heart, which is where a lot of blood jumps from the right side to the left side and bypasses the lungs, so it's not getting oxygenated," Horner told Cyclingnews Saturday at the race hotel in Knoxville.
"So literally I'm on the trainer with the doctors there, and as soon as I hit 300 watts, the [blood oxygenation levels] would drop into the 80s, and as soon as I went over 300 it would drop down to the low 80s. The doctor said, 'Oh yeah, you have a problem.'
"So I just spent six months trying to find the PFO leak, but we couldn't find the leak. Finally, I just said, 'Let's just solve whatever this gunk is in my lungs, because we know that's a problem.'"
Horner's doctor at the Jewish National Hospital in Denver recommended a test for Horner's doctors in his hometown of Bend, Oregon, to run.
"And so they ran a rod down the nose and into the throat," Horner said of the test. "Every 10 seconds you swallow water and they see if it flushes back up. And what they were seeing was that everything I was swallowing was just flushing back up, going into the lungs and causing an infection.
"They started me on the stomach med Pantoprazole. It's a medicated anti-acid reflux stuff that calms the stomach and the limits the fluids flushing into the lungs."
Horner has been on the medication for just over two months, and things seems to be returning to normal for the long-time pro.
"The first month of training was just a couple hours a day, because before that I was only doing 150 miles a week for the last two years since Lupus [Horner's team in 2016 - Ed]. So then I did one week of training at 250, one week at 275, one week at 300 and one week at 325, and I was feeling better and better," he said.
"This last month I've been doing training like at 550 and coming back each day and wanting to train more. I wasn't fatigued in the morning like I had been. For a year and half it was hard to get out of bed. I'd get out of bed and just be devastated. Once the doctor calmed the stomach down, now when I get out of bed in the morning, I'm like, 'Let's go train.'"
In a serendipitous bit of timing, Illuminate's Johnson called Horner earlier this month and talked to him about joining the team for several upcoming races.
"We talk a couple of times a year, so he knew I was bored at home," Horner said. "He called me up and asked me to come do a couple of races with the team. I said, 'Did somebody tell you I was feeling better?'
"He said he hadn't heard anything but that I should come ride with them. I said, 'Well, I'm actually kind of fit. Maybe I will.' I literally was wondering if my wife called him or something just to get me out of the house."
Horner speculated that his off-road motorcycle riding, which had increased in volume once he no longer was putting in long training rides, had his wife a little worried.
"I'm riding the dirt bikes all the time," he said. "I'm not good but I can do some jumps and some doubles. You're doing 40 miles per hour when you hit the jump, and then you're flying 40 or 50 feet. So I think my wife liked the idea better that I'd go out training on my bicycle all day versus getting on the motorcycle."
Horner will also ride the Tour of Romania with Illuminate, followed by a 14-day stage race in Colombia, both countries Horner has never been to before. He's looking forward ot the adventures.
"I would be at home and I would literally know the dog's routine," he said. "I know when he wants to eat. I know when he wants to go out. I know when he's got to take a shit or take a piss. I know the dog's routine, and I'm like, 'This is not what I was made for,' so I was looking for any kind of excuse to get out of the house, but I needed to feel good first."
With a new-found health, form, increased training load and power numbers, Horner said the decision to ride US Pro was an easy one, especially when he saw the course profile and the likely weather conditions, which could favour a rider of his size and abilities.
"Between USA Cycling, UCI, Medalist and Chris Johnson, we all worked hard on Monday and Tuesday to make it all happen," he said.
So with his team taken care of and eligibility intact because he never left the anti-doping whereabouts and testing system, Horner was ready to go. He just had to get to Knoxville. His original plan had him arriving in Knoxville at 10:30 Friday night. But when a delayed flight meant he'd miss the second flight, he had to rebook, flying a red-eye into Nashville, landing at 5:30 a.m. and then driving a rental car to Knoxville. He got in a little after 10 a.m. on Saturday. It's not ideal preparation for his first race in almost two years, to say the least.
"The year when Ben King won [2010 - ed.] I did a redeye that night and I got fourth that year, and I arrived the morning of the race," he said. "I don't really know how the form is but we'll see.
"I'm not going to outsprint everyone, but I can make it on the climbs, and so we'll see here on Sunday whether or not I've got enough form to follow the best guys on the climb and then have enough tactics, savvy-wise, and play for a little bit of luck too," he said.
"But I wouldn't be devastated if I got dropped and finished 20th or 30th either. I haven't raced in two years, so I would expect anywhere from a win to 40th."
Watch the USA Cycling Pro Road Championships on Cyclingnews. Live streaming will begin with the elite women's 115.8km road race at 9 a.m. and follow with the elite men's 193.1km race at 1:15 p.m. on Sunday.