Hey there, I answered your question on the Q&A thread, just letting you know in case you miss it :)
Does anyone know how many steps equals a pokemile?
Pokemiles aren't based on steps; Pokemiles are earned by trading with friends, passersby, on the GTS or Wonder Trade. You can then exchange your Pokemiles for items.
Source:
http://www.serebii.net/xy/pokemiles.shtml