I've been covering all the roads in my city as well. After several months and several thousand km of riding I think I only have 2 weeks left. Unfortunately I messed up my hip on the weekend so I won't be doing any riding for a while.

If you use Strava, this site will let you overlay all your rides onto a map. Strava Multiple Ride Mapping Tool | Strava Tools The actual application is a blue link in the first sentence. I find it looks better than the Strava heatmap (which is also only available to premium members). 18 months of files might be a bit much for the site.

For the actual tool I like to set opacity to full opaque, trace size to 1 pixel and use a single colour. One problem with it is if you have rides where you stopped your gps and forgot to turn it back on. A straight line will be drawn directly between the stop and restart point. The Strava heatmap manages to filter those out.

After a while you start to develop strategies for covering roads. When zig zagging through roads aligned in a grid, I plan it so I make right turns on the busier of the two end streets. When there are medians on both end streets I skip a road on one end and back track on the other so I can make right turns on both ends.

I picked a zoom level I liked then combined a couple print screen captures.