Ah yes, but bear in mind both that the Harrier pilot is managing an infinitely more complex aeroplane, but also that a lot of those systems are there to do much of the routine work - like navigation - for him (or her).
Personally I mark 12nm intervals, or 6nm in a particularly slow aeroplane. If one is over featureless terrain or I'm too busy I don't worry about it, but I can divide a time by 2, 3 or 4 easily enough in my head, and generally do most of my navigation in minutes per 12nm. I don't like time markers, since it's all too easy for en-route events to change that.
Works for me anyhow,
G