More than 170 teams are taking on other competitors and the conditions in Winchester for the 22nd Triple Crown Challenge.
Setting off from Worthy Down Barracks, they can choose either a march and shoot, or a military skills challenge.
Those on the skills course have to look after a battlefield casualty in their first challenge, before travelling four miles.
Those on the march and shoot have to cover 10 miles with 18 kilos on their backs, then undergo a test of their marksmanship on the 100-metre range, before a race back to camp.
Organiser Major Ian McLeish said it is a chance for those involved to "reunite, forge new relationships and also to take part in a really cracking training event".
However, for most of the 800 people taking part, it is more about teamwork than timings. As one of the participants put it:
"Sticking together as a team helps a lot."