Infantry soldier Josh Hoole who died while on a training exercise was conducting a 'standard fitness test', according to the Ministry of Defence.
The eight-mile march began at infantry centre Dering Lines in Brecon around 7am on Tuesday - which was the hottest day of the year so far.
Soldiers were carrying a total weight of 25kg and were wearing boots, carrying webbing, a burgen and a rifle.
The MoD says the march was 'basic combat fitness' and it is a 'standard minimum requirement test'.
The march was conducted along tarmacked roads along the Brecon Beacons. It is understood there were no hills along the route.
The march lasted for two hours and upon returning to the centre, Mr Hoole collapsed and an ambulance was called.
26-year-old Joshua Hoole died on Tuesday while taking part in a training course in Brecon.
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The Defence Committee has said it will be asking the MoD questions following the death of an infantry soldier in Brecon.