En in those sentences is a personal pronoun. It usually represents something that has previously been mentioned. The use of en is different in each sentence you give. We’ll have to add context to explain.

Et si vous en avez le courage vous rentrerez à pied.

En represents rentrer à pied. It could be dropped in that particular sentence without altering comprehension but most people would leave it. En is used here because avoir le courage would be followed by de (vous avez le courage de rentrer à pied), there’s a question about it here: Replacing phrase with "en"

– Est-ce qu’il y a des publicités dans ce magazine ?
– Vous en avez une (deux, trois, etc…) tout au début.

En is necessary when expressing a notion of quantity and no actual noun phrase is specified. It could not be dropped. In the above example, en represents publicité. In the same way we could add to the previous sentence: “… et vous en avez une autre à la fin.”

Both youre examples use avoir, but of course, the use of en is not linked to avoir.
(Si vous vous en rappeler rapportez moi des pommes. Il y a des oiseaux dans le jardin, j’en vois deux.)

<sub>You say you have been trying to translate into English, but one can never translate word for word. In the examples you give English does not translate en at all.
1. if you’ve got the guts… / if you feel up to it
2. There’s one right at the beginning.
But you’d translate en in Tu veux des biscuits ? oui, j’en veux bien. (yes, I’d like some)</sub>