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As mentioned, the Litmus hack it probably the best way to optimize your preheader. Keep in mind that having a short preheader and using the hack means there will be a lot of blank space, so it's something you might want to A/B test if having a shorter or longer preheader helps.
The prep alone will teach you a lot of things you didn't know about Marketo. I recently just got certified (after working with Marketo for 3 1/2 years) and it definitely makes an impact when working with various stakeholders. They'll see you as a subject matter expert, and this can be a great thing ...