Tips for preparing for my first 7-mile run?

I have been doing the same 3-mile run, once or twice a week, for a couple years. I just decided I’d like to run Bay to Breakers (a big fun run in San Francisco that is 7 miles long). The run is next week. Today I ran about 5 and a half miles just to see if I could do it, and it went well, although I ran very very slowly. I would run Bay to Breakers just to finish – not for speed. Any tips for how to prepare in the next week? Should I run a shorter course (say, 3 miles) a couple times, or not run at all and do yoga? what about the kinds of foods I should eat?