I did 16 days, 9 days too long. After a week the early mornings and same boring people made it suck. Never again unless free and I would want free flights too. Good luck for 18 days, hope the group is ok.
Did only 1 guided tour longer than 5 days, never again. Just far too structured sitting on buses with the same people and lots of early mornings. The best tours are no more than 3 days. No interest in Morocco or Oman. Looking at old stuff wears thin.