I am an Alaska Airlines Captain based in Anchorage and have flown that route about a dozen times. This is the longest route we have currently in our system, slightly longer than MIA-SEA, ANC-OGG, ANC-HNL and SEA-LIH. The longest "block time" that I have flown is approx. 6 hours and 45 minutes. Winds in the winter can be very strong flying N.W. We usually fly this with a 737-800, and all are the same "standard" model fuel-wise; however most of the fleet are also ETOPS qualified to support the approx. 110 flights per week to 4 destinations in Hawaii (HNL, LIF, OGG, KOA). When we began this route (2001 I believe) it was flown with a B-737-700 (before we had -800's) and we have also flown it with a -900. We are rarely limited weight wise and can carry full passenger loads as well as mail, cargo (and out of ANC lot's of fresh fish in season).
The most advantageous alternates for ANC are Elmendorf AFB (800 pound burn) followed by ENA (Kenai....1100 lbs.), FAI (3800 lbs.) and CDV (Cordova ...2900 lbs.). I have also seen Juneau as an alternate (JNU....7600 lbs.) and also, rarely, Whitehorse, Y.T.
We usually dispatch with more contingency fuel when flying ANC-ORD due to weather, ATC delays, etc. associated with ORD arrivals, even though the flight time is much less than ORD-ANC (with prevailing winds).
Hope this helps.
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