A team of Royal Marines will be running 16.64 miles a day for 100 days to raise money for the Corps charity.
The challenge will involve all of the Commando units, from Plymouth to Abroath, Faslane to Chivenor.
They've set off on the mission from Plymouth, from the same spot where the Pilgrim Fathers left in their pursuit of the new world.
Alongside the two core runners who will perform the entire 1,664 miles, there will be support teams starting off in Torpoint.
One of the participants, Cpl Jamie Thompson, said:
"I am just going to take each day, one day at the time, and hopefully just crack on."
The team has launched a JustGiving page to collect funds until the end of the 100 days, on August 2.
It comes as part of the Royal Marines 1664 Global Challenge, which will see servicemen and women performing 100 missions in 100 days to raise money for injured Marines or veterans.
The number 1664 has always a symbolic significance for the Royal Marines, as it was the year of the forces' birth.