Event spokesperson Jeff Darman initially told CNN that Tillis suffered a seizure and was transported to George Washington University Hospital -- but a spokesman for Tillis later clarified that doctors have confirmed no seizure occurred.
Tillis later tweeted a written message and video of him in a hospital bed.
"I'm doing well. Looking forward to getting back to work. Thanks for all of your prayers and well wishes. #CapChallenge," Tillis wrote on Twitter.
In the video, Tillis said he was "overheated" and that "no CPR" or "special measures" were needed. He did not say whether he'd had a seizure.
"Hey everybody. I'm fine. Just running about 2 and half miles in, got overheated, no CPR, no special measures, just checking me out," Tillis said in the video. "See you back on the Hill."
Tillis, 56, was first elected to the Senate in 2014, after serving as speaker of North Carolina's House of Representatives. He sits on several high-profile committees in the Senate, including armed services, judiciary and veterans' affairs.
Gov. Roy Cooper of North Carolina, a Democrat, tweeted well wishes to Tillis.
"My thoughts and prayers are with Senator Tillis and his family. - RC" he tweeted
His Senate colleagues also sent him prayers.
"Please pray for my Senate colleague @ThomTillis" Sen. Marco Rubio tweeted
Several members of Congress planned to run in the annual three-mile road race, including Georgia Republican Rep. Tom Graves, Sen. Tom Cotton of Arkansas and Sen. John Cornyn of Texas.
This story is developing and will be updated.