The 31-year-old has scored two goals in three appearances for the Three Lions so far.
"If I don't get to play for England again, I'll still be happy with what I have achieved so far," he told The Mirror. "This is already the best season of my career.
"Obviously, I don't need to tell you how determined I am not to let it end now but if it did then I am more than happy with the way it has gone. Personally, I feel I can improve even further.
"Being around the England setup will bring more out of me. I might be 31 but there is so much still for me to learn from the manager and the players."
Lambert scored 15 Premier League goals for the Saints last season.