Such is Francis Benali’s unadulterated cult-hero status that stickers of him have appeared everywhere from Bali to Budapest but on Sunday his adventures will be confined to a treadmill, with the former Southampton defender running a marathon at home to raise money for charity despite little to no preparation.

“I’ve seen one or two that are a little bit risque, shall we say, not too family friendly,” he says of the stickers, “but there have been some brilliant and wonderfully amusing ones like the Golden Gate Bridge, and places in the far east and Australia. They crop up in some incredible and unusual places.”

Benali is no stranger to gruelling challenges, having raised more than £1m for Cancer Research UK across three ultra-endurance tests, the most recent of which saw him complete five Ironman triathlons – swimming 12 miles, running 131 miles and cycling 560 miles – across seven days last year. At the end of day four Benali was broken, struggling to eat and drink and, physically spent, was withdrawn on medical grounds and taken to hospital amid concerns he was potentially inflicting lasting damage on his organs. He took two days off before being cleared to finish on day seven.

“My inability to consume the calories led my body to start crashing. I was determined to carry on and when you’re at that point of just wanting to continue, to get in the pool, or on the bike, or on the road running again. You can be so blinkered and turn tunnel vision on to try and keep going to achieve your goal. But it was like stepping into a danger zone. I certainly hope this marathon is not going to be anywhere near as painful as those challenges because I went to some really dark places.”

Francis Benali stickers in San Francisco, Bali and Budapest Photograph: @kevjudd1/twitter; @iamsmity/twitter; @Tkitchhh/twitter

Four years ago Benali was awarded the freedom of Southampton following the completion of his second remarkable challenge, whereby he ran or cycled 1,400 miles across a fortnight, visiting all 44 Premier League and Championship grounds, and in December he was awarded an MBE for his services to cancer patients. The investiture is scheduled at Buckingham Palace this summer but those three letters are still sinking in. “I don’t know if I will ever get used to hearing them. It was wonderful to be recognised in that way but I see it as a collective effort of my family and everyone that has supported me and donated, that led to that honour. It will be an emotional occasion.”

Francis Benali practises on his treadmill in the garden. Photograph: Courtesy of Francis Benali/Southampton FC

Inspired by the lockdown feats of Capt Tom Moore, the second world war veteran who walked 100 laps of his garden to raise more than £28m for NHS Charities Together, and James Campbell, the javelin athlete who ran a marathon in his six-metre garden, Benali planned to run 717 laps of his garden path but after completing 15 laps on Wednesday the 51-year-old reported feeling nauseous and dizzy, leading to Southampton’s director of performance science, Mo Gimpel, advising a change of plan. A modest character, iIt is not in Benali’s make-up to throw in the towel and his wife, Karen, suggested the treadmill as an alternative to complete a marathon in aid of the Saints Foundation and NHS Charities Together.

“The last thing I would have done was cancel it. It’s still going to be very tough and a pretty long slog in one position, having never run anywhere that kind of a distance on a treadmill in my life. Previously I’ve used a treadmill as a warm-up in the gym to have a quick blast and get a sweat on but the longest time I’ve spent on one is probably around 20-30 minutes. It’s going to be a physical challenge but I’ve been trying to prepare my mind by visualising being on a treadmill that long, and how mind-numbing it might be.”

On the face of it, 26.2 miles is light work given previous superhuman efforts but, this time, sparked into action by the achievements of Capt Moore, Benali has given himself little more than a week to prepare. On Tuesday he ran 6.2 miles, his furthest in the past eight months, and his priority is completing the distance safely.

Francis Benali battles with Manchester United’s Ruud Van Nistelrooy in 2003. Photograph: Danny Martindale/Action Images

“I’m not going to be breaking any sub-two-hour marathon records, that’s for sure,” he says, chuckling. “It feels very strange and different to how I would have approached a challenge normally – usually I have given myself a year-plus notice – but I guess that makes it exciting as well. I have to be sensible with my pace because of my lack of training but I’ll probably be looking at between four and five hours.”

His solitary goal for Southampton, in 1997 against Leicester, brought the house down at the Dell – “There was a stunned silence when the whole stadium thought: ‘Was it actually Franny that scored?’ I think I did the same, and then it went bonkers” – but now his primary focus is on taking part in the national ‘2.6 Challenge’ to help charities in the absence of the London Marathon, on the day the world’s biggest one-day fundraising event was scheduled. “My wife is going to be my chief supporter. She is going to have to act as a cheerleader, keep me refreshed, encourage me and capture some footage as well, so she’s probably got a bigger job than I have on the day,” he says, laughing.

• To donate to Francis Benali’s challenge, click here