Mike Buss, 41, from Highworth near Swindon, is accused of defrauding the charity Help for Heroes out of more than £23,000 between 2009 and 2011.
Swindon Crown Court heard before becoming an endurance athlete, Mr Buss spent 10 years in the Army and lectured at St Joseph's College in the town.
Mr Buss denies one count of theft and one of fraud by false representation.
The ex-Green Jacket's fundraising activities took place mainly in and around Swindon and Highworth.
They included running on a treadmill for 24 hours in Swindon's Brunel Centre, and running 100 marathons in 100 days.
Help for Heroes operations director Mark Collier-Elliott said when he first met Mr Buss he thought he was "an extraordinary man doing incredible things".
The court was told that during the time he was raising money for the charity, the former serviceman had no income apart from a small army pension.
Bank statements presented to the court showed Mr Buss was making regular payments of £500 and £200 to his ex-partner Kirsty Gorton during this time to help with her mortgage.
Colin Meeke QC, prosecuting, said Mr Buss raised "thousands and thousands of pounds" but was "subsidising his current account from his charity account".
The court heard he failed to keep proper accounts of his fund-raising activities, and named his charity bank account Beyond Impossible, in recognition of his numerous endurance challenges.
Mr Buss raised about £40,000 meant for Help for Heroes by collecting donations and selling official merchandise.
However, the charity said it only received monies totalling about £17,000 from him.
The trial continues.
A former soldier asked for donations for endurance challenges but kept money raised for himself, a court has heard.