The players are driving and really control the pacing the most. However, a GM can switch gears or hit the breaks by choosing a twist or a condition for failure. If the GM takes away a backpack or blows out torches, the party is going to want to get those back at some point, so you need to account for that happening.
Start at the Door
For the first adventure, start right at the dungeon door. Get rid of the stuff leading up to the dungeon for the first adventure. Get crawling!
Later, getting to and from town is an important consideration for the dungeon’s difficulty. At mid-levels, half the fun is even getting to the dungeon. At higher levels, make the players work for it. By the time they get to the dungeon, they’ll have to make some hard decisions. Also, getting back to town has to be an important part of the tension and the party’s calculations to make it back safely. At low levels, getting home could be a Pathfinder test, but at higher levels, there could be a few obstacles on the path back home or even at the gate of the town.
Areas & Sessions
In general, the average is about 3-4 areas per session (sometimes just 1 or 2).
A typical mid-level adventure might have between 7-10 areas. A high-level adventure can have up to 20 areas (and take 6 or 7 sessions).
Town phase could possible take an entire four-hour session if you have a party of 4 or 5 players with almost all of the conditions to alleviate (otherwise a town phase might average about an hour for an experienced group).