Undead movement rates

My players are currently being pursued across the wilderness by a quartet of mummies.  How does one scale up the combat movement rate for beings such as undead or golems which do not tire nor need to stop for rest.  Mummies, frex, have a movement rate of 60, which according to the chart on p. 93 translates to 12 miles *per day*.  But this number is, I believe, essentially assuming an eight hour adventuring day.  Should mummies be able to travel 36 miles per day, without stopping, or is the assumption that they travel 12 miles over the course of their "day" (in this case 24 hours)?

Yes - my understanding is that those movement rates are on an 8 hour schedule; as implied from the "12 hour force march" rule. So, mummies moving 12 miles/8hours would do 26 miles per full day.

I'd see no reason any undead not bothered by sunlight wouldn't be on the move the full 24 hours - the only caveat I'd think of is if they'd swerve to avoid settled areas.

Also, I really want the backstory there.

The party has been steadily chipping away at an ogre village over the past two weeks of game time, to the point where they've reduced it down to two and a half warbands from ten.  Unbeknownst to them the village lies ontop of an ancient burial complex, after the latest sortie the ogres decided enough was enough and bribed four mummies within the complex to chase down the party.

Luckily, the party assassin was in the complex, invisible, and saw this, so he was able to escape ahead of the mummies and warn the party that was camped about ten miles from the village.  They have since retreated to the Keep they're rebuilding and are preparing for a potential mummy attack.

The party is around 5th and 6th level, with oodles of henchfolk.