Question is how to match up Google Maps with tiles.
I guess first thing is to find the right "zoom" level, as in, metres to tiles.
Then rather than drawing the world too accurately, I'd try and work out whether a particular tile, like an OS grid square, is mostly grass, water, trees or road.
Minor roads, bridleways and major roads would all be differentiated. Perhaps Motorways would be lava, as you can't walk on them.
Then I'd draw tiles based on that, with some "autotile" type blurring of the edges to look a bit more natural.