This wouldn't technically be a mileage run, because MRs are trips you take solely for the miles. But padding your trip with extra connections is a time-honored tradition for MRers and non-MRers alike! However, every connection increases the chance of a misconnect.

Status makes a huge difference on this, because if you're Gold or higher, you can do a same day change for free. That can really help if you see that your connecting flight is going to be delayed. You can try to get back on that direct flight and get there on time, or if you've already connected, you can get on a flight that is technically scheduled later but will take off earlier.

For December, I'd say SFO is the safer bet than DEN, mainly because of the number of scheduled flights -- both from SMF to SFO vs. DEN and to LAX from SFO vs. DEN. You also don't have to worry about a winter storm in DEN. SFO's notorious fog is far worse in the spring/summer than the winter, and runway repairs should be finished by then.