Rooney was sidelined for the festive period after suffering a knee injury in training but has returned to score three goals in his last four United games.
He told The Sun: "I had my injury problems early on in the season and did a lot of hard work to make sure that when I came back into the United team I'd be fully fit and ready.
"When I had my injury over Christmas and the New Year I still did not have a single day off.
"I went into training every day and worked to get back to my best. And now I feel it's paying off for me."
Rooney also revealed that he asked England coach Roy Hodgson to let him play 90 minutes against Brazil on Wednesday so that he could stay sharp.