The 22-year-old will travel to England's training camp in Austria next week after being named in Fabio Capello's 30-man pre-World Cup squad but will have to impress the Italian if he is to make the final cut for South Africa.
Johnson, a £7million January signing from Middlesbrough, admits his selection has surprised even him but now he has been given an opportunity he is optimistic about his chances.
"Just being in the 30 means I have come a long way," said Johnson, who is likely to make his international debut in either the Wembley friendly with Mexico on May 24 or against Japan in Austria the following week.
"It's now up to me to prove in the two weeks away what I can do. If I get a call-up to the final 23 it would be the best moment of my career and cap an incredible six months for me.
"Hopefully I might be in with a shout. Stranger things have happened."