Negotiation Blunder

I just received a job offer of 55K base at the low end of the salary scale and though I knew that the range was from 53-68k when I was asked by the hiring manager if I was willing to accept I said yes. Now I am thinking that I should have negotiated. Is it too late..I told her I would get back on Monday with start date. She did talk about review but it was vague and said that I should be in a senior level in no time she anticipates.