Always far easier to say than do… Anyone knows the power behind HERE app or other major offline navigators? Hint: behind Kurviger app there is just 1 person. 
For the record, Cruiser platform (where Kurviger is based) is perfectly capable to do offline routing (process explained above).
Seems GraphHopper (used by Kurviger) has its limits in offline large scale routing, so would need to check also other solutions, like BRouter discussed here.