The desert in question isn't all sandy dunes; there's a lot of rocky or compacted surface (see photo), so you can make pretty decent time (but yeah, sustaining 4mph would be pushing it for most people).
I've done 97km in 26 hours before, in the Sahara (I think we started around 8am, and finished around 10ish the next morning), which works out to about 2.3mph, so I'd agree 2-2.5mph is reasonable.
hmm, [0] this says the longest stage was 91km in 2009 (I did it in 2015), so I could be wrong, but I'm going by what my garmin told me.
