Mr Thrall said: "After leaving the Marines, it can be a big shock for some people.
“If you add to that a traumatic experience, then it extenuates the confusion of the situation.
“When you start relying on drug and alcohol to release the stress, it could end up compounding.
“It is a change of culture. There are different rules and a different set of ethics outside the military.
“It can be very hard to come to terms with - the lack of depth which passes for OK in civilian life, the lack of people who have got your back, being in the workplace with people treating you like dirt.
"Military people make up an unusually high proportion of people sleeping rough, maybe more resources need to be put into that."
Mr Thrall said he "nearly lost everything" in Hong Kong and that he "completely lost" his mental health.
He hopes his '999 Mile Challenge' will help others, with all funds being donated to the charity 'Bastion Baton'.
Before embarking on the run, Mr Thrall had undergone four operations on his back and says he was “pretty much disabled” for two-and-a-half-years.
He said: “Everyone is telling me to stop with the run and to rest...but I'm not stopping.