If you haven't already changed out the OEM front rubber brake lines to the caliper, do that first... Even if they 'look' fine and don't leak, when you press on the brake, they actually expand and you don't get as much pressure to the calipers. I changed them on my old 97 F150 and it made a huge difference in the braking power.
As to how many miles you get out of a set, your running loaded most of the time is gonna wear them quicker for obvious reasons.. You might just have to try various pad materials and see what's going to work best for your application. I've heard good and bad things about the ceramic pads. I'd try a semi-metallic brand after the new brake lines and see how it goes..
Good luck!
Mitch
