MrsG; challenges indeed
I echo what has been said above. The communication part especially times a million; you're embarking on new territory here, and it is going to take time to learn even the basics. Be honest and open.
I'd also like to add on a thought; have you met your husband's girlfriend? I realize such a meeting might be tension filled, but I think it would also help to smooth waters from many perspectives.
My wife has a boyfriend. I've met him on many occasions, and I consider him a friend. I wouldn't want him to be an anonymous person to me.