Good news is that you don't need to prepare to general GRE in the same way as to subject GRE. Q section is quite easy for physics students, consisting mainly on high school math. Verbal section can be substantially improved with practice, but it is not that important in admission process.
I have the impression that very top schools require high scores in other parts of the general test as well. For instance, University of California at Santa-Barbara has high requirements in all parts of the general test ... .