It's a great way to raise money for charity and stand out in the crowds, but London Marathon runners planning to don fancy dress are being urged to reconsider their outfits this year.
The advice comes amid warnings the race could be the hottest on record - as temperatures could hit 23C (73.4F) on Sunday (April 22).
The warm weather forecast has prompted organisers to add more water, ice and shower stations along the 26.2-mile route.
The warning came on the hottest day of the year so far, with 25.7C (78.3F) recorded at Heathrow Airport on Thursday (April 19) shortly before midday, the Met Office said.
Goodbye winter blues!
Conditions may be especially difficult for fancy-dress runners, including the almost 100 attempting Guinness World Records dressed in outfits like a suit of armour, a Paddington Bear costume and ski boots.
Regular runners are being advised to consider dropping their goal-times and run more slowly.
Adam Smith, who is attempting the record for fastest race dressed as a mythical creature, told the Press Association: "My costume is particularly warm, with a nice and furry gnome hat and a toadstool to carry.
"The plan is to go on as normal but I've asked if I can have short sleeves - I'm very conscious that there's a world record and then there's being safe.
"I suspect there won't be a lot of people doing PB's (personal bests)".
The 45-year-old lawyer, who has run eight marathons, is raising money for muscular dystrophy research, having lost three family members to the disease.
The hottest London marathon on record was 22.7C (72.9F) in 1996, the Met Office said.
Marathon event director Hugh Brasher said: "Most importantly, runners should adjust their goal for Sunday and plan to run at a slower pace.
"If they were planning to run in fancy dress, they should think carefully if that is appropriate in the forecast conditions."
"We want to stress that there will be plenty of water available and runners should drink according to their thirst and use spare water to douse their head and neck."
Met Office meteorologist John West said: "Potentially we're looking at highs of 23C but we could potentially see some showers coming in from the west which could take a degree or so off.
"Showers could be a bit of a double-edged sword because it could make it that little bit more humid for runners."
There will be 39 first aid stations dotted along the route that runners should visit if they feel overly tired or unwell, organisers said.
Water is available every mile from miles three to 25, while six shower stations can be run through to cool down at miles nine, 13, 17, 20 and 22.
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