After years playing in the lower leagues, Lambert's consistency for Southampton was rewarded by a first England call-up last August.
Lambert seized his opportunity by scoring the winning goal in a 3-2 victory over Scotland at Wembley and he has been included in all the subsequent squads since.
But Lambert believes he still has much to prove to manager Roy Hodgson ahead of the finals in Brazil, especially with West Ham's Andy Carroll regaining his match-fitness.
And the 32-year-old hopes he can get another chance to impress in Wednesday's friendly against Denmark at Wembley.
"Obviously, I still understand it's nowhere near secured," Lambert said. "I've got two months to try and prove my place. It's as simple as that.
"I've always said if I don't make it I'm going to be grateful that I got my chance, and that's still the case now.
"I'm obviously desperate to go and now I've got two months to prove that I'm good enough to go, so that's all I'm trying to focus on now.
"I understand that I've got to do well. I've got to do well in every training session, any minutes I get. I understand that, it's fine.
"I relish that, to be honest, and that's what I'll be looking forward to. It's going to be great. It's going to be enjoyable."