The 35-year-old told BBC Three Counties Radio: "This is one of the most important years in a footballer's career with the World Cup and a potential Asian Cup in Australia.
"There's a lot of exciting football I want to play in."
"From my point of view it's not a fitness thing - I've been training for six weeks now," added Neill, who was a free agent when he signed a deal until the end of the season at Vicarage Road.
"There's a few players in the Australia squad who are in leagues such as the MLS or in China which haven't started yet.
"Rhys Williams at Middlesbrough has left a big hole in our defence and we have a lot of untried and untested players.
"The manager has this game (Ecuador friendly) only to work out what sort of defenders he is going to take to the World Cup and he's probably seen enough of me to know what I can and can't do.
"The message to me was to get used to the environment at Watford and push for a position as soon as possible."
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