Jones started slowly as he adopted the “couch to 5k” approach.
“As soon as I started doing it, I saw the benefit of the process. It was fascinating to see and develop week-to-week,” said Jones.
“I enjoyed that and started ticking off the little milestones. It is a brilliant process.
“The first run was a five-minute walk, then a 90-second run, and so on. You’re not going straight in and doing it.
“Once you’ve got that 5k, it was a sense of achievement and you get that buzz after running.”
He took to his new sporting endeavour so well he soon upgraded his ambition.
“After that, I was thinking of doing a 10k, but then I saw Cardiff was long enough in the distance that I could train for a half-marathon,” he added.
‘Weirdly, I enjoyed the running part of training, so it appealed to me.”
Jones recently did a 17km training run, not something he did during his 16-year rugby career.
“It’s definitely the longest I’ve run,” said Jones, who with typical humour went on: “If you added up all the distances I’ve run in Test matches, it probably wouldn’t be far off that anyway!”
Jones says Harlequins players and coaches have been supportive.
“The boys have been brilliant,” he said.
“A few of them have sponsored me. Some haven’t, but that’s mostly the big earners.
“I think they’re waiting to see me finish first and then the money might go up a fair bit.”
The former prop also believes it is a positive for the players to see him making the effort.
“I’m putting myself in a place where it’s uncomfortable.
“I’m asking them to stay in the fight and stay in shape when it gets painful, so it felt wrong if they couldn’t see me do the same.
“I do have perspective. It’ll be hard, but there’s harder things to do.
“I always tell the boys here you’re not working down a mine. My grandfather was a miner. What he did was hard.“