Royal Parks Half Marathon organisers have urged runners to “have fun and enjoy your experience” rather than chase a personal best amid the unseasonably warm weather which is forecast for London on Sunday.
Temperatures may reach 24C (75.2F) during the 13.1-mile event which passes through Hyde Park, The Green Park, St James’s Park and Kensington Gardens.
The average temperature for this time of year is around 18C (64.4F), the Met Office said.
In a message to participants, organisers said: “We want you to have fun and enjoy your experience running in the beautiful Royal Parks.
“It is not worth chasing a PB in these unseasonably warm conditions.”
Around 16,000 people are expected to take part in the event, with the first runners due to cross the start line at 9am.
Participants have been urged to use sunscreen and to make sure they are hydrated before they start the race.
There will be four water stations with compostable cups along the route but it is also suggested people carry their own fluid in a bottle, waist pack or hydration vest although, on social media, event organisers said: “We have ensured our stations will be sufficiently stocked for runners.”
Met Office meteorologist Craig Snell told the PA news agency: “Sunday is likely to start off with a mix of some cloud and also some sunshine across London with perhaps also a little mist or fog around for the early arrivals.
“Throughout the morning the cloud and mist will begin to break up and by lunchtime there should be some prolonged spells of sunshine across the London area.
“Throughout the race it will remain dry with a light south-west wind.
“Temperatures at the beginning of the race will likely be around 15C and by the end of the race reaching 23C or 24C.
“The average for the time of year is around 18C.”
The Royal Parks charity looks after Hyde Park, Kensington Gardens, St James’s Park, The Green Park, The Regent’s Park and Primrose Hill, Greenwich Park, Richmond Park and Bushy Park.
It also manages other important green spaces in the capital, including Brompton Cemetery and Victoria Tower Gardens.
The Royal Park Half Marathon began in 2008.
This year’s event is supported by the Royal Bank of Canada, Sweaty Betty, Tenzing, JustGiving and JW Marriott Grosvenor House London.