"Such stations must be located in places that are not regular station locations and must not use facilities installed for permanent station use, or use any structure installed permanently for Field Day."

I interpret this as meaning a portable station is any distance from the in-home radio shack and not using radio equipment  "installed for permanent station use" (such as radio shack installed stuff, coax in the ground, regular permanent antennas,  etc.)  

I will be operating QRP under battery power in my driveway using a portable antenna and portable transceiver and associated non-permamently installed equipment.  That would be  the 1B-Battery class in my calculation.

Unless you have a ruling from ARRL that talks about a "fair hike distance from the home", as you put it, for a portable location, I think stations using portable battery power, portable antennas, and portable gear, etc., regardless of  distance from a house with a permanent  and separate station, are in fact impermanent and, hence, "portable.  

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I do not think the phrase "must not use facilities for permanent station use" includes the bathroom and other non-radio facilities  because they are not installed for the purpose of permanent radio station use.  

I think the rules on portable stations is intended to eliminate from the "portable" category those big gun, permanent contest stations built specifically to house radio  equipment and permanent antenna farms.   The purpose of Field Day, in part, is to encourage portable operations of various kinds (sources of electrical power, transmitter output powers, number of operators, modes and bands, etc.) that could be used in emergency situations (most eminently when there is no access to commercial power).  

Jim, n0ssm