If it's a,myth it's fiction. It never happened. There is a genre called historical fiction. I've read those books. Very fun reading. But even though they are based around a time that acually existed the story itself is fiction. Greek mythology is based on Greek times, but the story is not true.
Saying Genesis is myth is saying that in history there may have been a time of beginning in history, but the story itself is false. God may have created the heavens and earth, but he did not do it the way the Bible claims he did. In fact the bible says nothing about how he did it. Just that he did. So the bible isn't being honest with us about how God did it, it's being deceptive.
The purpose of the,bible is to give us the history of the world, particularly focused on Israel and Gods working in the lives of his people and what happened to them. Including the giving of the law. A strict history books purpose is to give strict history and that's it. The bible is history with a purpose. The history is to show the ultimate purpose of God. That man is fallen and in need of repentance and redemption. See as God inspires it, it's not just some dried up history textbook you get in school. Its actual factual history of God moving on this earth, creating, choosing his people, providing the law, leading, guiding, blessing andbpunishing his people and ultimately providing Jesus to,save us.
Whatever was written in the past was written for our instruction so that we could have hope through endurance and through the encouragement of the scriptures.
Romans 15:4 CEB
http://bible.com/37/rom.15.4.CEB
But you must continue with the things you have learned and found convincing. You know who taught you. Since childhood you have known the holy scriptures that help you to be wise in a way that leads to salvation through faith that is in Christ Jesus. Every scripture is inspired by God and is useful for teaching, for showing mistakes, for correcting, and for training character, so that the person who belongs to God can be equipped to do everything that is good.
2 Timothy 3:14-17 CEB
http://bible.com/37/2ti.3.14-17.CEB
Brothers and sisters, I want you to be sure of the fact that our ancestors were all under the cloud and they all went through the sea. All were baptized into Moses in the cloud and in the sea. All ate the same spiritual food, and all drank the same spiritual drink. They drank from a spiritual rock that followed them, and the rock was Christ. However, God was unhappy with most of them, and they were struck down in the wilderness. These things were examples for us, so we won’t crave evil things like they did. Don’t worship false gods like some of them did, as it is written, The people sat down to eat and drink and they got up to play . Let’s not practice sexual immorality, like some of them did, and twenty-three thousand died in one day. Let’s not test Christ, like some of them did, and were killed by the snakes. Let’s not grumble, like some of them did, and were killed by the destroyer. These things happened to them as an example and were written as a warning for us to whom the end of time has come.
1 Corinthians 10:1-11 CEB
http://bible.com/37/1co.10.1-11.CEB