Are you staying at Tempe Mission Palms? If so, there is no need to drive a car anywhere - you are within .25 mile from the light rail and about .75 mile from the finish line.
The light rail travel is a new occurrence for the marathon this year, and really is the only option with the buses not being used. If you are staying at TMP, I would recommend catching the light rail from the station right by the hotel. It is supposed to be free for athletes between 5-7a on race day.
I would not recommend parking at the start, as it will not be cheap, and getting back there will require a light rail or taxi ride, neither of which will be free post-race.
If you are not staying at TMP and have a car, I would recommend parking at one of the Arizona State University parking structures within .5-.75 mile from the finish, as they will be easier to get in and out of than will the parking areas right at the finish line (if those are even open). There is a parking garage just south of Sun Devil Stadium, and my wife always parks at a different parking structure on McAllister Ave, just south of University (about .4-.5 mi from the finish line area). Rates are pretty cheap, I think less than $10 if I recall (I am never there when she pays, obviously). If you park at either of the garages, there are light rail stations close to both.
Whatever option you choose, give yourself some time, as with the half marathon starting from Tempe now, you can bet the traffic into the area will be heavier, as there will be none of them getting dropped off downtown this year - they will all converge on ASU.
The light rail is supposed to make things easier this year, and with only marathoners going to downtown Phoenix (about 6-8K max), accessibility to the rail system should not be too bad. But again, give yourself plenty of time, unless you thrive on running late and enjoy the stress of pushing time to the limit. HA HA