"Just wanted to let everyone know I'm doing ok and taking one day at a time. Once my brain has had time to rest, I'll be having face surgery to repair my broken [cheek]bone."
The ironman legend also revealed how he felt after the accident.
"I feel so, so lucky to be alive and the accident is still under investigation. Please be safe out there.
"I'll keep everyone posted with my progress and plans. Much love.''
He accompanied the message with a photo of himself running towards a finish line, with an arm up punching the air.
Police said on Wednesday that they believed they had tracked down the man thought to be the driver of the truck involved.