McLaren driver Alonso shocked the Formula 1 paddock when he announced he will miss the Monaco Grand Prix in order to compete in IndyCar's blue riband event next month.

The Spaniard began his preparations by visiting the IndyCar race in Alabama last weekend, before heading to the Andretti team base in Indianapolis and then running in the Honda simulator.

While the news had a huge impact in F1, Alonso said he was surprised by the scale of the reception he received in the States.

"I walked in the airport in Indianapolis, in Chicago, and everyone is stopping me and shaking my hands, wishing me good luck, so it was like being in Spain for a moment," said Alonso in Russia on Thursday. "Definitely a big change in America for this race.

"I saw also the audience on television was 50 percent more share on that race, so definitely a bigger impact than what I thought in the beginning.

"As I said, the biggest surprise of this weekend or the last five days was the impact of this news and how many people recognise me in those moments, and in the airport.

"I am used to walking in America quite relaxed and I used to spend holidays in the US because no one recognised you and you had some privacy, and it was not like that in the airports at least."

Alonso admitted he had much to learn when he visited the IndyCar race at Barber Motorsports Park.

"Everything is different there," he said. "The paddock, the people, the fans around the car, around you, the way you interact with the team, with the competitors, the drivers' briefing. I was invited to the drivers' briefing, and there were things that were definitely different compared to here.

"It was quite an intense weekend, then from there I flew to Indianapolis for a seat-fitting on Monday with the team to go through some of the stuff on preparation for the race, and on the Tuesday, I think, I was into the simulator in the morning just a couple of hours, three or four hours, also to have the first touch on those cars, on the ovals.

"Of course the real car will be very different, because in the simulator it is easy to go 370km/h and not lift in the corners because nothing will happen. I think the real car will be a little bit more difficult, but at least I had the possibility to see the car to see the efforts that you need to do in the corners."

Alonso says that his Indy 500 programme had "reset" his mind and will benefit him when he returns to his day job in F1 this weekend.

"I think in a way it helps in terms of learning new things," he said. "So you reset a little bit your mind, and you are again a rookie in a sense, so you start learning things and being very open to engineer discussions and things like that, and driving techniques in a simulator.

"So you arrive here with a motivation to do well, hopefully finish in the race, hopefully score the first points of the season. But definitely the days between races are quite busy now so it is good for the head."

Alonso said he has already began to build a relationship with team boss Michael Andretti, the man who will be speaking to him on the radio during the race.

"He was very, very warm with us, welcoming us in a proper way from the first day," the Spaniard said. "He organised a mini-barbecue for us, we arrived with all the drivers, he has so many experiences, the Indy 500, and so many good moments, that he will be a fantastic help.

"He will be running my car on the strategy. I talked to him a lot, we had every opportunity, lunch, dinners together, on those days, and he's been a big part of the project so far."