The A-T Society, a charity based in Harpenden, is organising a "MegaTri" and urging locals to join their A-Team.
Inspired by Lola Bloomer, 5, who has the disabling and terminal condition Ataxia-Telangiectasia, the event will be held in Brighton to take advantage of the Brighton marathon.
The Mega-Tri will start with a 4km swim on the morning of Saturday, April 11 in Brighton University pool. This will then be followed at midday by a 100km bike leg, a route taking in some of South Downs countryside and quieter roads, and will finish at Brighton Pier.
The next morning, the Brighton marathon will take centre stage with the A-Team flying the flag for every child with A-T.
Fundraiser Suzanne Roynon said: "Individuals and relay teams can take part in the swim and cycle legs and we have individual marathon places. You can do all three disciplines or just one or two.
"It's an event for all ages, and if the last one is anything to go by it's going to be a lot of fun."
To take part email info@atsociety or call 01582 760 733.
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