It is a test of physical and mental strength.
But only those with the toughest willpower and determination will be able to take on Only the Brave this year as the event returns to the Elveden Estate, near Thetford, to raise crucial funds for charity.
Organisers promise that this year's course - due to be held on Saturday, April 1 and Sunday, April 2 - will be 'bigger, better, and muddier than ever before'.
Participants can choose between a five-mile or 10-mile mud runs with over 40 obstacles, meaning those taking part will have to run, crawl, climb and scramble their way to the finish line.
Last year more than 1,300 people from across the region took part and raised more than £100,000 for the East Anglian Air Ambulance (EAAA), to help volunteers keep saving lives across the region.
The charity's doctor and critical-care paramedic teams treat patients with life-threatening or life-changing emergencies at the scene of an accident or emergency, before transferring patients to the appropriate hospital rapidly.
It operates two dedicated air ambulances 365 days a year and has a fund-raising target of just over £11m this year.
The last date for entry is Wednesday, March 22 - so if you're brave enough to enter, now is not the time to lose your nerve.
This year Norwich-based IT company Breakwater will be taking part along with Aviva, which will be entering a team of more than 100 people.
Lowell Black will be entering a team for the second year running, while City College Norwich has a team of students entering the race.
Students are also volunteering across the weekend to gain some valuable work experience.
Go Ape will be supporting Only The Brave and is offering an exclusive 10pc discount on entry to its facility at Thetford Forest to all participants. Look out for the Go Ape group on the day to receive your exclusive offer.
Thorpe Travel will also be sponsoring Only The Brave, as EAAA is its chosen charity of the year. Barnwell Print, Sportspark UEA, and Devour are also amongst the event's supporters for 2017.
To sign up, visit www.eaaa.org.uk/events
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