The Ukrainian, who will be 41 when he enters the ring, will battle the undefeated 27-year-old for the IBF and WBA belts on April 29.
Klitschko, whose rematch with Tyson Fury was called off twice last year, has not fought for the past 18 months but is relishing the challenge that awaits him.
"I honestly asked myself, 'Do I really like what I do?' and I said 'no Wlad, I actually love what I do'," Klitschko told Sky Sports News. "This is one of the biggest challenges I've had in my life - coming back after an 18-month break and fighting a young champion, really talented, athletic and all of this.
"I'm the underdog in this fight and this is actually exciting for me. I haven't been in this situation for such a long time. I feel young and challenged. I'm the official challenger - great.
"I want to leave the ring as the champion and the winner of this fight - that's my obsession. I'm obsessed about it."
Joshua has won all 18 of his professional bouts by way of knockout.