I tend to use intermediate way points if using Directions on Google Maps.
If going from Mackay to Cairns, I'd break it up into stages such as Mackay to Bowen, Bowen to Townsville, Townsville to Innisfail, Innisfail to Cairns. That way Google traces the path you want and you can get intermediate distances and times as well.
If Google shows a path off from the path you want to take, you can hold your cursor on the path and move it to the road you want to take.
Ask for a trip from Mackay to Townsville, Google Maps will give you the coast road (Bruce Highway). If you wanted to go inland and up to Chartres Towers and out to Townsville, just place your cursor over the shown route (a dropdown box saying "Click to move route" will appear), click and hold on your cursor, and move the route to a road you want to use.
EG: click on the blue route shown at Bowen and move it south and west to the next north/south road and release, the route will now show SW out of Mackay, north to Chartres Towers and east to Townsville.