Taking the 90 Day Challenge: How do I tell her?

Go after what you want and don't care about what she is going to think about it.
If you want to be on hard mode for 90 days, just do 90 days hard mode. If she wants to have sex before that just tell her that you are not allowed to do that until the 90 days are gone because you are practicing semen retention because of all the benefits you read about it. You don't have to tell her you are doing it because you where a porn addict, but you have to stick to your guns.
If she wants to have sex with you she is going to go at you with all that she can to make you cave. Woman love a challenge, and a man that don't want to have sex with them is a challenge woman are not used to have, and find it really attractive. Keep your word no matter what, even if she said that she will leave you if you don't have sex. if that's the case, tell her to contact you if she change her mind or after the 90 days are over.