Crosswell said he wanted to lead by example and show his grandchildren that anything is possible.
"Exercise is such a vital part of life. It requires discipline and it gives a sense of purpose.
"I hope when they look at me they'll be able to say, 'Poppy's still going'."
The 73-year-old said he will be easy to spot in the crowd of runners. He plans on wearing a bright purple T-shirt with the words, "Sweating for mental health", written across the back.
Crosswell is running the marathon to fundraise for the Mental Health Foundation. He hopes he won't be the only one.
"Though my life, running has really lessened stresses. It's been really good for my mental health. I suppose my message would be, 'You can do it'."
Thousands take part in the Rotorua Marathon each year. As of Friday there were 2375 participants registered to run the 2022 marathon, with more registrations expected on the day.
Traditionally the marathon has been held in May but was postponed to today's date due to the global pandemic.
The historic Rotorua Marathon has been a regular fixture on the annual running calendar for 58 years and this is only the second time the event has been postponed.
In 2020 the event was postponed because of the global pandemic with the rescheduled race run later in the year. The event was cancelled in 1999 when flooding washed out part of the course.
Rotorua Marathon road closures:
Thursday, September 15, 2022, midday-midnight:
Queens Drive (Government Gardens) will be closed from the roundabout towards Hatupatu Drive for 100m.
Queens Drive road closure will be extended from the roundabout towards Hatupatu Drive for 200m for the Mini Marathon.
Saturday, September 17, 2022, midnight–6pm:
Queens Drive road closure remains in place.
Additional road closures include:
• 6am–8pm: Queens Drive road closure extended to the roundabout.
• 7.30am–10am: Arawa Street from Hinemaru Street to Ranolf Street(Lake side only), Fenton Street from Arawa Street to Whakaue Street (Lake side only), Ranolf Street Arawa Street to Lake Road (Lake side only), Parks Road entrance off Ranolf Street (Lake side only), Lake Road from Ranolf Street to Railway Road (Lake side only). (These roads will be progressively opened as walkers pass through from 8.10am)