The Tottenham winger set up Wayne Rooney for England's opener in Friday's clash with Montenegro and then scored a stunning third himself, driving straight at the defence before producing a brilliant finish from the edge of the area, to secure a 4-1 victory.
The triumph left Roy Hodgson's men needing just one more win - against Poland in their final qualifier on Tuesday - to clinch an automatic place in Brazil next summer.
Speaking to www.tottenhamhotspur.com, Townsend said: "It hasn't sunk in at the moment and I haven't allowed it to because we've got another big game on Tuesday.
"That's the best thing, to keep my feet on the ground and try not to get carried away."
Townsend, who tweeted a picture of himself holding the man-of-the-match champagne alongside the words "the perfect debut" on Sunday, admitted he could hardly have wished for more.
"No matter who I'm playing against, I look to play the same way," he said.
"I look to take people on, make things happen and try to get my shots off. It all came together. It couldn't have gone any better."
Townsend has been in fine form for Spurs this season, having established himself as part of Andre Villas-Boas' plans following several stints away on loan - most recently at QPR last term.
Reflecting on the moment he was told he would be rewarded for his efforts with a start against Montenegro, he said: "I found out lunchtime on the day of the game.
"The manager came to me a few times during the week and said he was undecided in that position, a few players were vying for it. I'm so grateful I got the chance and I'm pleased I was able to repay his faith in me.
"I didn't really think about it, about how big it was. I was excited to get my first start and was determined to make the most of it.
"It was only afterwards that I thought how big a game it was for England - if we hadn't won it we wouldn't have been in prime position to qualify automatically for the World Cup and that would have been unbelievable."