As mentioned above, the Rogue River Trail would be good (although it is only 42 pretty fast miles) as would any of the 50+ miles of the Olympic Coast. Hell's Canyon is spectacular and could be outstanding in March.
Your not going to get any distance on the PCT even in April or May.
I don't know much about "the Desert Trail" that is supposed to have 100 miles or so of beautiful trail completed in SE Oregon and may have enough trail low enough (i.e. not on the Steens Mtn) to be passable in March.