We stayed at the Holiday Inn Express. Less than a mile walk. They do offer a Shuttle Van, but it doesn't go to Petco because of the gridlock, as they put it, we'd never get the shuttle out of there.
I did not know how hilly San Diego is. It is a straight across to Petco park, no hill. But it is very hilly up and back from the waterfront, we did use the shuttle for that.