Interesting find. A few comments:
- The route map between Nairobi and Khartoum shows a straight line. I'm guessing they flew for political/logistical reasons. This region is mostly Sahara desert areas (or Ethiopian highlands if they went a bit further east) as well as politically sensitive. After that perhaps following the Nile and/or on the water e.g. Lake Nassar?
After this discussion, I bought this as a used book online. It took a while to reach me since the seller was in the UK.
I haven't read it yet, but can confirm they flew from Nairobi to Khartoum. They followed the Nile river valley after that.
This makes sense as Southern Sudan, Ethiopia and Northern Kenya were some of the toughest bits in when I rode in 2013 - and this is after a lot of new paved roads have been built that wouldn't have been there in 1988. So the other bits would also have been considerably tougher in 1988...