United kick off their new Premier League campaign against Swansea City on Saturday, having appointed a new manager in Louis van Gaal and brought in summer signings such as Herrera following a miserable seventh-placed finish last season.
Herrera came close to moving to Old Trafford last summer but a deal collapsed as Bilbao turned down United's offer.
United returned with an improved bid in June and the 25-year-old Spaniard is relishing the challenge of playing for a club he sees as 'perhaps the world's biggest club'.
He told the Guardian: "You go to the US and there are a thousand people outside the hotel. You play Real Madrid and there are 109,000 people, almost all in United shirts, 80,000 against LA Galaxy, 60,000 against Roma.
"Every day at Carrington there's someone else I haven't met yet. You get a sense of the magnitude of the club. You go into the dressing room and see Fletcher, Carrick, Rooney, Van Persie, guys who have won it all and been at the highest level for 10 years, not two or three."
For the first time in 25 years United will not play in Europe this season, and Herrera is desperate to ensure it just a one-off.
He added: "I would never have left Athletic if it hadn’t been for a club like United. I left a different, unique club, with a special philosophy and incredible people.
"It's a club that few of us have had the privilege of playing for and, who knows, but for United I might have been there 10 years.
"But this is the biggest club in England, a new project, new players and a new coach. Desire too: it's been 25 years and the players are conscious of that. They're determined to put United back where it belongs."
"I'm 25 and I thought it was the right time. I wasn't thinking: 'The train's leaving, the train's leaving … ' but United is an opportunity you can't turn down."
When Herrera almost joined United in 2013 there were confusing reports of imposters who claimed to represent the Premier League club meeting with Bilbao officials to try and get a deal done, but he insists it was not that complicated.
"Much more was said than really happened," he said. "From my point of view, it's simple: United made an offer to Athletic, I told the clubs to talk, I would listen and Athletic turned it down. United offered a lot of money but not the buyout clause. Athletic said no and that's it."
Herrera added: "In Spain, players have an official price tag, just like shirts hanging in a shop do. It wasn't a difficult decision. A year on, the move happened.
"I hadn't been wondering what might have been or worrying over whether they'd return. It was going from 'very, very good' to 'excellent'; it wasn't escaping a bad situation.
"I was happy in Bilbao, so my thoughts were on enjoying playing for Athletic … we made it to the Champions League, so it's safe to say I enjoyed it and gave everything. I understand that there are people who didn't like me leaving but most understand that this is United."