Alex Paget, from Clapham, will run the 26.2-mile race on April 27 dressed as a giant pink brain.
The handmade costume, made of PVC pipes, cardboard, expanding foam, and fabric, took him 25 hours to create.
The 30-year-old insurance analyst aims to become the new Guinness World Record (GWR) holder for the fastest marathon runner dressed as a body part.
Alex is running the marathon to raise funds and awareness for Headway, the brain injury association, a charity that provides support, services, and information to brain injury survivors and their loved ones.
Avid runner Alex knows all too well about life with a brain injury, as he lives with post-concussion syndrome (PCS) following an accident on his bike nearly four years ago while taking part in an Ironman challenge in France.
Alex, who lives with fiancée Laura and dog Toby, was 90 miles into the cycling part of the long-distance endurance race, which comprises swimming 2.4 miles, cycling 112 miles, and running 26.2 miles, when he fell off his bike.
He said: “My wheels just came out from under me, and I went down and apparently hit my head.
“I didn’t know that there was anything wrong with my head, but my shoulder was pretty messed up.
“I was very annoyed because I thought ‘Oh this has ruined my race’.”
Alex ended up completing the race but began feeling unwell over the coming days, including getting headaches, brain fog, vomiting, and feeling dizzy.
The following years have left Alex with PCS, a debilitating continuation of concussion symptoms, with severe fatigue, constant headaches, and cognitive issues leading to a yo-yo cycle of having to take time off work, often for months at a time.
He has also had to start from scratch with his exercise, something he found difficult to accept as an Ironman competitor.
Alex said: “The last three and a half years have been a big adjustment to the pace of life and what I can do.
“I think a lot of people don’t understand what you’re going through.
“They can’t really comprehend why you’re so tired all the time or why you bail on plans last minute.
“I wouldn’t wish what I’ve gone through on my worst enemy.
“The support I’ve had from my loved ones has been amazing, especially from my fiancée Laura.
“I don’t think I’d be here if it weren’t for her support.”
Researching became Alex’s new hobby, which led him to find a concussion programme as well as Headway.
He said: “A doctor told me about Headway, and I found some of the booklets online, which are really helpful.
“Headway were there for me throughout my recovery, and I want people to know about the charity and what they can offer in way of support.”
Alex is aiming for a Guinness World Record in April’s London Marathon, hoping to secure the title of fastest runner dressed as a body part, beating the current male record of three hours and 43 minutes.
Alex said: “I’ve wanted to do the London Marathon for a long time, and I wanted to mark my return to ‘normality’ with something fun.
“Every kid got given the Guinness World Record books, and you’d flick through it and see ‘fastest marathon runner dressed as a knight in shining armour’, and I just thought that would be cool to do it but in a brain costume.
“I’ve done three runs in my costume so far.
“People find it very amusing, and I’ve had lots of horn honking.
“Even if I don’t end up getting the record it’s about raising awareness of concussion and brain injury, and Headway.”
