I surround my original stronghold first then take the nearest stronghold to the west of it while filling in the blank spaces in the North West until I reach what would be Seattle.
I then move down the West Coast and start back East. By the time I reach Houston I can start occasionally skip to reinforce.
I'll make 2 large groups and continue moving East through the south and middle of the map. I'll take all the South East strong holds and then just skip to reinforce moving up the East Coast taking two or three strongholds a turn.