Sadly, I don't think there are options to help with this. I hope I'm wrong but I believe the brush size is limited to the maximum slider size and the terrain view distance can't be increased. I tried to see how fast I can paint a flat 4096x4096 terrain and it took me several minutes to awkwardly fly around and paint, area by area, sometimes not being sure what areas I've already covered (since you can't see all of the terrain). This is after I sped everything up to maximum under Tools menu, Options, Viewports, Camera section and I held the Shift key to move around as fast as possible. Working with this map size is very difficult.
That said, consider that perhaps your map size may be too large. I remember Jen thought that she would need a huge map and I think she said 512x512 was still too large for her project, after some further thought. Consider putting some temporary landmarks at the edges of the map and see how long it takes you to get there with reasonable speed, even in a vehicle.