Since the OP's question was dated back in mid-summer of 2012, things have changed a bit with Maps and has been updated since.
In agreement with other answerer's who have pointed out you can download the maps for offline usage, no calculation is needed, if using Maps, what I found actually surprised me, the GPS navigation still works even if off-line and not using any 3G/Wifi either!
With the maps, hit menu, tap on Make available offline, then you can pan and zoom in on the location of the map, to increase the size of the map for offline usage, it may be required to zoom out if you want a big area of maps coverage.
Then the download will happen (WiFi needed preferably as depending on the size of the offline map, it will take a while - not your conventional nippy download speed)
Furthermore, if browsing on desktop pc, hit up on maps on the google site, and select your starting point and ending point as destination, if planning a journey, and save that as "My Places". With the maps app, it can sync up with "My Places" and pull it down and be used with the off-line map! That surprised me
The GPS navigation will actually show where you are and the route to take.
Caveat Emptor: Be sure to have plenty of battery juice as the GPS is working 100% on it
Simply enable GPS, all data switched off, and as its oft said - Let's hit the road Jack and... profit! \o/