This is one of the longer walks, 3400 meters in total. It'll take between 45-60 minutes, depending on the gear and pace.
As for Fortress but continue along path for another 25-30min after the Fortress turn-off.
You'll walk up to a big plateau and just when you feel that the area cannot be here and you should just give up, the cairned trail to the left will be in just a couple of steps away.
Fear not, you don't walk too far. There's a clear junction of 4 paths if you walk too far. If that happens, go back maybe 200-300 meters and you'll find the correct path =)