The London Marathon is the ultimate test of endurance and super-fit runners will be trying to finish the 26-mile course today in the fastest possible time.
There are several sporty members of the royal family who could take part, however only one has actually completed the London Marathon - and not only did she finish the route, she also set a record.
That royal is Princess Beatrice, who ran the race in 2010 for her mother Sarah Ferguson's charity Children in Crisis.
Beatrice was one of 34 runners who joined together to form a human caterpillar and were tied to each other while running the route.
Among the others taking were part were her then-boyfriend Dave Clarke and friends Sam and Holly Branson.
Beatrice donned a fluorescent tutu and sweatbands for the race and was pictured cheering with her medal after crossing the finishing line.
And impressively, the group broke the record for being the most people to finish a marathon while tied together.
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However, today, it is likely Beatrice will give the marathon a miss and be at home with her husband Edoardo Mapelli Mozzi caring for their two-week-old daughter.
One royal who is unlikely to do the marathon any time soon is the Duchess of Cambridge.
Sporty Kate is often seen at engagements trying her hand at all sorts of activities from running to tennis - and even abseiling.
But it seems that taking part in the famous long-distance run in the English capital is one sporting event that she won't be taking part in, even if she wanted to.
Journalist Bryony Gordon, who has met Kate and completed the marathon, revealed she asked her if she would consider doing it.
But Bryony told Runner's World : "I can’t remember exactly what happened, but I was like, ‘Are you going to run the marathon?’
"And she was like, ‘Oh I can’t because security is a bit difficult.’ But I was like, ‘Sure, yeah, yeah, good excuse. If I can do it, anyone can.'"
Despite not being able to take part, Kate did attend the 2017 London Marathon when her mental health cause Heads Together was one of the main charities for that year - although she didn't do any running.
