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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.
The soldier who died in Brecon while on a training exercise has been described as a 'beautiful grandson' and a 'superfit boy' by his family.
Josh Hoole, from Ecclefechan near Lockerbie in Scotland, was a member of The Rifles regiment.
The 26-year-old, who died on Tuesday, was conducting pre-course training for the Platoon Sergeants' Battle Course, which is described as "both mentally and physically demanding".
His grandfather John Craig said he was a 'beautiful grandson.'
"He was a dedicated soldier. He always wanted to be top dog. He was a superfit boy, he kept very fit."
"He visited me regularly, every chance he got. I took a bottle of whisky to him on the 12th of July for his 26th birthday."
Police say they are investigating the circumstances surrounding the death of a soldier in the Brecon Beacons.
The infantry soldier died on the morning of Tuesday 19 July - the hottest day of the year so far.
An infantry soldier has died while on a training exercise.
The MOD said the soldier, from the Infantry Training Centre, in Catterick, North Yorkshire, died in the Brecon area of Wales on Tuesday, Britain's hottest day of the year so far.
Three soldiers died during an SAS training exercise in the Brecon Beacons on one of the hottest days of 2013.
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