It's a very well documented fact that for most recreational runners doing their first longer distance race, walk run intervals are very good. It helps you to hold form and recover faster. And as others have stated, many people will actually have a better time when they run this way. This does not hold true for higher trained athletes.
As for interval work, fartleks are great during base building and it's ok to do intervals after a couple of months of base training. You don'thave to wait a whole year. IMHO.
The suggestion of doing some hills is a great one. I hadn't thought of the San Fran factor. For the first 4 weeks I'd suggest doing the hills at a low intensity (ie very slow, conversation pace if possible) and then build some intensity in for months 2 and 3.
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