Moistness in terms of mouthfeel is mainly about fat, you've got the least fatty part of the animal. Broken down connective tissue (collagen) will also contribute to a moist mouthfeel, you have the muscle with the least collagen.
Collagen at sous-vide temperatures takes days to break down, there's not much difference between three hours and six. The cooking time in this situation is mainly about the thickness of the meat (how long the centre will take to get to temp) then further time if you want to pasteurise the centre.
Not sure how you got nasty and lard in the same sentence.
If it's rubbery (too firm?), then drop the cooking temperature. 137°F is a good start.
Modernist Cuisine provides the following temperatures for pork loin:
pink: 60/140 <-- Preferred by MC team
There's other tables for how long various sises and shapes take to reach core temperature, they assume you set the water bath 1-2°C above the target. Below is from 2.277 Table 2, Cylinder shaped meat with a 15 cm length. MC has mistakes in the imperial conversions on this table so below I have only written metric.
10cm / 55°C = 4:28 hrs
7.5cm / 60°C = 2:50
10cm / 60°C = 4:28
10cm / 65°C = 4:31
I hope this helps