Fletcher endured three years of pain and frustration after being diagnosed in 2011 with chronic bowel disease ulcerative colitis.
But after a string of false starts, he has played a pivotal role in pre-season, clocking more minutes on the pitch than any other member of United's 26-man squad, and captaining the side to an impressive warm-up win over Real Madrid in Michigan.
"I don't want to look back, but everyone knows I've been through a lot," Fletcher said.
"I have not had a pre-season for three years but now I am back in the gym and I am feeling the benefits from that.
"I am getting stronger and hopefully I can get as fit as possible. I am looking forward to the new season.
"Liverpool, Manchester City and Chelsea all finished above us last year but now we all want to show what we are capable of."
Van Gaal is the man tasked with turning United back into title contenders after last season's seventh-placed slump under David Moyes and Fletcher admits the Dutchman, despite demanding double training sessions as he implements a new 3-4-1-2 formation, has lifted the gloom.
"We have got a fantastic team spirit," Fletcher added.
"We were talking about it the other night among ourselves. We are all pulling in the right direction and we all want the best for Manchester United.
"The manager expects a lot from us and he thinks we have got the ability to do it.
"He demands a lot, but the players have responded.
"We are enjoying playing in a new system and under him."
Watch Manchester United take on Liverpool in the International Champions Cup final, live on Sky Sports 1 at 12.30am on Tuesday.