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Joshua, 26, has won all of his 15 professional fights by knockout.
Joshua told BBC Sport: "[Mike] Tyson took 28 [fights before becoming world champion], [Muhammad] Ali 20, I think Lennox [Lewis] was 24.
"Some will say it's too soon, [but] opinions are irrelevant - I have to do what I feel is right."
Martin, who beat Vyacheslav Glazkov for the vacant IBF belt in January, is unbeaten in 24 fights while Joshua most recently beat Dillian Whyte with a seventh-round knockout to claim the British heavyweight title.
"Experience is valuable," Joshua conceded. "I've been in the Olympics, big British dust-ups, I've gone through these feelings, these emotions, these expectations.
"The reason I'm here so soon is because of what I had to go through early on in my career, and what I have to take with me on 9 April [is] that belief in myself that I'm here, I'm capable and I'm ready to rock and roll."
Martin is unbeaten in his 24 fights but believes he has been denied the adulation that Joshua has received from the boxing world.
"I want what he's got - the praise, the attention, and the fans," the 29-year-old Missouri native said.
"I don't take the glamour and all that stuff. I don't soak it in, I just go back to that tough grimy kid from the block, from the boxing gym, trying to make a way. That's what I do in preparation - go back to the roots."
Martin said Joshua had faced easy opponents in his professional career but admitted the same charge could be levelled at himself.
"The past is the past," Martin said. "The record, putting [Joshua] in with guys who were tailor-made for him to lay down - we've both had those people in our past and that doesn't matter. What matters now is there are two hungry fighters in there wanting to win."
Anthony Joshua says he is "ready to rock and roll" when he fights American Charles Martin for the IBF world heavyweight title on 9 April.