How do I come back from low-balling myself during a salary negotiation?

I was offered a management level promotion at my company, which is one step above my current role. HR just called me during a busy time and offered me $65K but being flustered I countered at $70k. HR said they would call me back regarding $70K.

The problem is that I did my research weeks in advance and found that the market value for this role is $80K. In addition, I may or may not have been told by someone in senior management that the first offer would be a low-ball and to prepare to negotiate. I feel like a moron since I did my research but I froze under pressure.

Is there any way to come back from this and ask for $80K? I'm in the manufacturing industry and the job market is strong in my area.

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