The Rugged Road by Theresa Wallach (2nd edition intro and appendices by Barry M Jones).
I am sure you must have read it, two Women in late 1934 riding a Panther combination + trailer from Ealing to Cape Town. Including riding across the Sahara before it had been mapped. No help or back up, they didn't even have a compass. Great book, love it.
As an aside, I loved Jupiter's Travels but really couldn't get into the sequel at all. Seemed so totally different in style, could not finish it sadly.