Owen Willis, 49, is attempting to bag the Guinness World Record for the "fastest marathon dressed as a three-dimensional plant (male)".
Mr Wills, from Scarborough, must complete the 26.2-mile race in under five hours to take home the record.
He spent weeks designing and making his poppy suit, which weighs almost half a stone and comprises of 16 pieces that slot together.
The keen runner is raising money for the Royal British Legion and SSAFA, the armed forces' charity.
"When I ran my second marathon in 2014, there were all the poppies at the Tower of London," he said.
"I had this idea that in four years time I would do something to commemorate the end of the First World War."
Mr Willis began running in 2009 and completed the York Marathon in 2013 and 2014, when he ran it in three hours and 31 minutes.
"I would like to break the record," Mr Willis said.
"I don't know how many other three-dimensional plants are running - it could be a proper race. I would love to do it in under four hours."
Mr Willis works as a technical officer for White Rose Home Improvement Agency, which implements home adaptations for disabled people, including ex-service personnel and their families.
His great uncle, Digby Duggleby, died while serving as a sergeant in the London Irish Rifles, aged 27 in 1917. He is commemorated on the Menin Gate in Ypres, Belgium.
Simon O'Leary, assistant director of fundraising, at the Royal British Legion said: "We're thrilled that Owen is raising money for the Legion with his Guinness World Record attempt and wish him the best of luck.
"We've had runners dressed as poppies before, but never a suit of this size. Being so restricted will be a significant challenge."
Mr Willis is hoping to raise Â£5,000 for the two charities and his fundraising page is https://www.justgiving.com/companyteams/BigPoppyMarathon.