"Every time the pedal comes up, push it back down and you'll get there."

RICHARD SIMMS laughs nervously as he recounts the mantra. Makes it sound simple, doesn't it? As easy as riding a bike, in fact. Just keep pedalling. As he settles on his saddle at midday today, the 30-year-old will mutter the words to himself before easing on to the road and out of John o' Groats.

Time and again over the following 24 hours, the aphorism will rattle around Simms' mind. At times encouraging, at others almost goading, the phrase will accompany him for 300 miles and 30,000ft of pedalling though darkness, howling wind and maybe even snow as he and four friends cycle through Wick, Inverness, Fort Augustus, Fort William, Glencoe, Crianlarich, then down past Loch Lomond before arriving at Glasgow's Hilton Hotel tomorrow lunchtime. "At which point, one of the boys has said he's going to throw his bike in the Clyde," says Simms.

Little wonder. It is difficult to comprehend the sheer scale of the charity challenge that the five men – Simms, Stephen Keiss, Stephen Hogarth, Michael Martin and David Robb – are taking on, the numbers so large as to become divorced from reality, but consider this: the quintet will need to average a speed of 15mph to complete the 298.9 miles in their stated 24-hour target.

Granted, that affords two or three hours slack to allow them to eat and carry out any repairs but no time to sleep. "Most people think we're mental," concedes Simms. "I've actually had some tell me it's impossible and others, people who maybe go out at the weekends and do 15 miles in two or three hours, get a shock when they realise how fast you'd need to go to cover the distance in 24 hours.

"The time limit is the big thing. Every one of us know we are capable of doing 200 miles because we've all done it before but once you get to 200 and realise there is another century to go, that's when the mind games start. Most people in their lives won't cycle 100 miles so to do that three times in the space of one day . . . let's just say we're all going to hurt like hell and be in a very dark place at one point but we've just got to get through to the other side."

To do so, they will have to temper their competitive instincts and work as a team. There will be no prizes for King of the Mountains, fastest on the day and quickest top speed, the onus instead being placed upon taking turns to lead the pelaton while the others shelter from the elements amid the pack.

Fortunately, each of the riders has experience of cycling long distances, having been part of a nine-strong group who pedalled from Lands End to John O'Groats in 2010, raising £30,000 for CHAS and the Highland and Strathcarron Hospices, and also been involved in the Audax UK club, which specialises in rides over 100km. "We did a few to challenge ourselves and that's when you end up coming up with these stupid ideas," explains Simms, the manager of the LivingWell Health Club at the Glasgow Hilton. "I was just sitting at my desk thinking about this thing in America called 'ride across Iowa in a day', which is 300 miles, so I checked how far it was from John O'Groats to the club. Bad idea."

Making the best of a bad situation, the quintet decided to use the cycle to raise funds for the Hilton in the Community Foundation and their nominated charity for 2012, the Blue Triangle Housing Association. The past few months, then, have been split between fundraising and training, with Simms amassing around 180 miles a week by cycling to work from his Falkirk home every couple of days and spending his days off last week, alongside Robb – his counterpart at the Strathclyde Hilton – cycling to Glencoe and back.

Having been a fitness instructor in his youth before graduating through the ranks, Simms is no stranger to exercise and has been cycling almost since he could walk. A racer in his younger days, he stopped when his work required him to teach spin classes and only rediscovered the motivation to return to the road a couple of years ago.

"You've got to motivate your members and by doing this they see that it is possible," he says. "We wanted to do something stupid so it would get some attention and people would think 'that sounds like torture'."

Simms confesses that the stretch from Glencoe to Rannoch Moor is the part he is dreading – "it'll be dark, cold, we'll be about 160 miles and having to climb six miles" – but such fears have driven him on to ensure he has the miles in his legs to meet the physical challenge. Surmounting the mental test, though, is another matter. "With 100 miles a day you know you're going to get there by mid-afternoon but, when we get to that point this time, we know we've got to do it again, then again after that.

"As one of my friends put it, 'every time the pedal comes up, push it back down and you'll get there.'"