They say write what you know. This is what I know. Hopefully, this does not turn out to be desperately boring.
You are a project manager of some experience.
Which is why you are worried
Your company has asked you to “just get us started” on a substantial project, though the final contract terms have not been settled.
Madness
You have warned them that this is a bad idea, but management insists. They explain they are confident that you can handle it.
You know the odds are not in your favor
Project managers can ask, but they cannot compel. Their greatest strength comes from speaking truth in meetings.
Document everything.