Thomas spoke of the 2025 ride and recalled one steep 18-mile climb in the Alps which took him to his limit, particularly when it suddenly started to rain and then snow.
“I was about two turns away from the top and I was crying my eyes out,” Thomas said. “There was nothing there but this pain.”
But he said it felt like he had “people on his shoulder” to support him, including some he knew who had lost their lives to leukaemia.
“They’d lost their battle but they were with me in that moment as I got to the top,” he added.
He said after starting the descent, wearing coats provided by the support team, the sun came out “all of a sudden”.
He said: “I just thought ‘wow, if I can do that, I can do anything’.”
Thomas, 61, was appointed an MBE for his work to raise money for cancer charities in 2021.
The Tour 21 challenge, which is 2,200 miles (3,540km) in total, is held every year with the aim of raising £1m for Cure Leukaemia.