Ross,
Nokio's post on p22 gives you details of how long each test lasts.
Perhaps NATS don't give you s/d/t calculations in the prep material to see how your mental arithmetic is without practice, and as for the distances between major cities, they might just want to see if you have some common sense and some idea of the world (you don't need the exact distance, just something in right order... ie. is LHR-JFK 100, 500, 1000 or 3000 miles). Just my thoughts of course - I don't work for HR.
Remember that this job requires some ability to be able to respond to the unexpected - treat these tests with the same thoughts...
Good luck!