By Angus MacSwan

RIO DE JANEIRO (Reuters) - Erika Villaecija did as thousands of Rio de Janeiro’s residents and visitors did on Sunday afternoon – she went to the city's famed Copacabana Beach.

But while most drank coconut juice and beer, played games or splashed around the water's edge, the Spaniard did a quick change from her Olympic athlete's uniform and went for a 15-minute swim round the bay.

On Monday, she will be back in the water taking part in one of the Olympics' most grueling competitions, the 10km marathon swim, which will take place in the bay near the historic Copacabana Fort.

Villaecija is competing in her fourth Olympics. Marathon swimming was only introduced in the Beijing Games in 2008 – before that she took part in pool events.

"There were head waves, it was choppy, the water is 21, 22 degrees (about 70 degrees Fahrenheit). For me that's OK but for others its cold," she said, toweling down at a beach post serving cold beer and snacks after the swim with her coach Francisco 'Kiko' Hervas.

Her competitors were training at the swimming pools but as her hotel was a block from the beach, she took advantage to get the feel of the course, she said.

The 32-year-old from Barcelona, who trained to be a psychologist, is Spain’s most experienced long-distance swimmer.

The event is rooted in the tradition of swimming the English Channel and now has its own world circuit, stretching from Scotland to the Pacific.

It has not previously been popular in Spain, but enthusiasm has grown in the past two years, particularly the run between the islands of Mallorca and Menorca, said Spanish swimming team physiotherapist Monica Solana Tramunt, also at the beach with Villaecija.

Unlike in pool swimming, wind chop and surface currents play a big part. The winner is the first to touch the end buoy.

"The winner will be the one with the better strategy and who can stay to the end," Villaecija said.

She did not expect to win a medal but was aiming to better her position of seventh in London in 2012. There, the race took place in the Serpentine lake in Hyde Park.

"It was like a swimming pool, here's its open water. I prefer swimming here than on a lake," she said.

The event will showcase Rio's natural beauty, with Copacabana's long, sweeping curve of sand and a view of the Pao de Acucar, or Sugarloaf mountain.

Villaecija said she will not be thinking about that though.

"In an Olympic race, you get motivated, you want to do your best," she said.

(Reporting by Angus MacSwan; Editing by Bill Rigby)