Kate Middleton, Prince Willliam and Harry at the London Marathon: All the highlights

Run for a cause! Prince Harry, Prince William and Kate Middleton were filled with joy as they attended the Virgin Money London Marathon on Sunday. The royal trio was casually dressed in matching black ensembles that featured the logo of their mental health campaign Heads Together. They offered words of encouragement to the 700-plus Heads Together runners who were set to take part in the race, which saw over 39,000 runners competing in total. Heads Together was chosen as the official charity for the 2017 London Marathon. Photo: Getty Images

Princes William and Harry got into the spirit as they watched and cheered the runners together. Photo: Getty Images

William showed off his playful nature, blowing a red horn at his wife Kate. Photo: Getty Images

Honk if you love the royals! Prince Harry and Kate played around with the horns as well. Photo: Getty Images

Harry and Kate showed their special siblings-in-law bond during the event. Prince Harry is patron of the London Marathon Charitable Trust, while the Duchess was attending her first Marathon in a royal capacity. Photo: Getty Images

The royals, all sporting matching Heads Together tops, were in charge of kicking off proceedings. Photo: Getty Images

That water girl looks familiar! William stood next to the Duchess as she stood prepared with water for the runners. Photo: Getty Images

The Duke and Duchess seemed particularly impressed with a runner dressed as a star. Photo: Getty Images

Prince Harry shook hands with a runner sporting the Heads Together headband as the Duchess applauded. Photo: Getty Images

You're my superhero! Spiderman stops for no one – except the royals, of course. This runner took a moment to high five the Duchess mid-marathon. Photo: Getty Images

His royal dampness! As the royals watched from the sidelines, Prince William got squirted by a cheeky race-goer – who was later revealed to be his little brother Prince Harry. Photo: Getty Images

Prince Harry was in full cheerleader mode, waving his Heads Together foam finger. Photo: Getty Images

Kate congratulated a marathoner who ran for the Heads Together charity campaign. Photo: Getty Images