In Lydia’s letter what joke does she include that makes it hard to write?

Lydia's joke is a secret marriage and change of name..... unfortunately, the joke is on Lydia, for Wickham will not marry her without prodding.

"You will laugh when you know where I am gone, and I cannot help laughing myself at your surprise to-morrow morning, as soon as I am missed. I am going to Gretna Green, and if you cannot guess with who, I shall think you a simpleton, for there is but one man in the world I love, and he is an angel. I should never be happy without him, so think it no harm to be off. You need not send them word at Longbourn of my going, if you do not like it, for it will make the surprise the greater, when I write to them and sign my name 'Lydia Wickham.' What a good joke it will be! I can hardly write for laughing.