The striker has been recalled to the Three Lions for the first time since 2013 after scoring 14 Premier League goals this season, despite the Black Cats battling relegation
Defoe could feature in Wednesday's friendly in Germany ahead of Sunday's World Cup qualifier against Lithuania at Wembley.
He wants to make Gareth Southgate's squad for next year's tournament in Russia - should England qualify - but conceded that would be in doubt if Sunderland were relegated.
"You have to be realistic, to get into any squad is based on merit and to do that you have to be playing at the top level," said Defoe, who has scored 19 goals in 55 games for England.
"When I went to Toronto (in 2014) I missed out on the World Cup
When I got the call it was hard to accept but you had to be realistic and think the boys I was competing against were playing the Premier League and Europe and that's why I missed out.
"So, yeah, I would say you would need to be playing in the Premier League but we'll see what happens.
"I know how important it is to play for England
Just going to a major tournament, travelling, getting to the hotels, playing the games - it's something you've always wanted to do from being a kid, growing up watching the great players over the years.
"I've been lucky enough to have experienced it
If I do get an opportunity to get to another major tournament it will be a dream."
Defoe's last cap came against Chile in November 2013 but the former Tottenham striker insisted he never gave up hope of representing his country again.
"I hoped not, but there were times where I thought 'maybe I'm not going to get in'
It didn't stop me from getting my head down and not giving up," he said.
"I was getting a lot of stick from the lads at the training ground because I've still got the England flag on my boots
I was hoping one day I'll get the opportunity and it's come so I'm delighted
"On a personal note if I'm feeling sharp I believe I can score goals
In this team I believe there will be a lot of chances to score goals
Nothing changes, if you get a chance you have to score
Simple as that."
The 34-year-old is joined by uncapped James Ward-Prowse with the England Under-21 captain having stepped up to the senior squad for the first time.
The Southampton midfielder has impressed this season and worked under Southgate when the England boss managed the under-21s.
Ward-Prowse said: "It's obviously an incredibly proud feeling for me
It's always been a life-long ambition and a career ambition to reach the senior squad.
"I think it's every young player's dream to play for England and to be given that opportunity and chance to represent your country and do everyone proud is a great honour.
"Should the chance come this week, I'll be ready to take it
I can't wait to draw every ounce of knowledge from other players who have been in the group before this week, to help me as a player as well."
Saints team-mate Nathan Redmond, another yet to make his senior debut, has also moved up to the senior side.
"I couldn't stop my smile (when he was called up), it was one of those moments where I couldn't wait to call my mum and immediate family," said the forward
"It just seemed a bit surreal, for the rest of the day, and it was a dream come true
"When I phoned my mum she started screaming down the phone, she was like 'finally you did it'
Being in the Premier League for the last couple of seasons, it's been one of my goals to play consistently and get into this senior side.
"She was ecstatic for me but at the same time was saying 'it starts now, you've worked hard to get here and now you have to work hard to stay'.
"To see so many faces I have seen in the under-21s is a good feeling
I'm here on merit, as the manager has said, and hopefully I can show that."
Source : PA