and even assuming the whole tunnel is difficult terrain because of squeezing or something, the trek should still only take 8 hours.
When I discussed this issue with my wife, she took it to mean that the journey takes 14 hours
including stops/breaks for the scripted encounters in each of the special areas along the way
and factoring in random encounters. You could also argue for areas of difficult terrain, as mentioned above (squeezing in certain tight tunnel areas, etc.). So, 14 doesn't seem so ludicrous if I factor in all of the encounters and exploration along the way.
However, there is obviously some lacking narrative if this was the intent of the authors. Again, something that should have been highlighted as a concern when playtesting a published module. All it would have taken is a slight tweak to the text (e.g., "The entire journey takes 14 hours, factoring in exploration of named special areas, encounters in those areas, and random encounters").
As it stands, I'm wondering if I'm supposed to roll for random encounters 14 times (since it says to roll once per hour) or if I'm supposed to roll for random encounters once per hour at the normal travel pace (which would equate to about 4 rolls).
As a DM, I'm probably going to rule somewhere in the middle, where I roll for random encounters at approximately the halfway points between key areas (e.g., Halfway between the entrance to the tunnel and Grell Caves, then halfway between Grell Caves and the Behir lair, etc.). I feel like that should allow the party to engage in a decent number of random encounters without slowing the pace to a crawl by rolling
14 times.