You've got a big job ahead of you, but it also sounds like you're up for the task. In fact, despite the stress you're feeling from your new position and the responsibility for developing a marketing plan, it's a fantastic opportunity.
Every marketing plan will be unique, since you're company, clients, market niche, budgets, and local market will be different from other insurance companies. It may be helpful to review some of the information found at bplans.com's sister site mplans.com.
I did a quick search for "insurance" and found these results:
There are some things there that may be helpful for you. Reading through some of the other insurance-related sample plans might give you some direction and a framework to start.