You can still gets maps on your phone. Go to
www.bing.com/maps and the Bing Maps app will open on your phone (protip: Get one of those free custom tile makers from the store, create an icon, and link it to that same URL. I prefer Bing Maps to Nokia Maps so I do it that way).
You can also just hit the search button, Local Scout, and click on the portion of the map that is showing. Until the update can complete or you can get it to complete, you'll have to use the Bing Maps app (all searching from the phone uses it anyway, so you're not totally disabled). Bing Maps also directly links to Nokia Drive if you use that... just hit 'Get Directions' and once they show, hit the bent arrow at the bottom of the screen.
If the download shows still (maybe as "Attention required") you can always tap and hold, then cancel the download and try again (if you haven't already). You can go to purchase history from your phone and try downloading that way, or try fully uninstalling Nokia Maps from the app list and try again completely.
PS: Apps are backed up and fully auto reinstall upon restore (I just did a restore - swapped out my phone due to a battery issue). App settings are a different story, but the apps are all there. I made a thread with a bunch of screenshots of how it all worked in the Lumia 810 forum if you feel like checking it out. I realize backup/restore isn't an option for you, but just pointing it out for the future if you swap out your device for another or whatever.