My 2015 Trek Verve has its original rim brake pads (Tektro). They stop well, and I have them adjusted so that they mostly do not squeal. The issue I experience with them is that, after about 10 seconds of sustained light braking (like to maintain speed down a hill), they become noisy, with a rather loud grinding sound. It sounds as if they're getting heat glazed and lose some effectiveness. I have taken them off the bike more than once, and washed them, sanded them down to remove any glazing, etc.

Is this behavior typical or expected for rim brakes like these? Is it a simple case of I need new brake pads because these have gotten hard or maybe because they're a poor compound? Or is my technique wrong? We have some fairly long hills and it's at least once a ride where I'm coasting down a hill and finding that I use the brakes (even riding the brakes?) to maintain my speed.

I appreciate any and all suggestions. Thank you in advance!