Rear tires do must of your braking. If you hit the front break too hard/fast, you flip over the front tire, hit the rear brake hard/fast and you burn rubber/lose traction.
Weight is transferred to the front on heavy braking, and if you have proper technique, you will not go over the bars. The front tire does all of the actual stopping.
When you initially hit the brakes, your braking potential is split around 50:50. At absolute maximum potential braking force, your rear tire is near lifting, so it's around 100:0.
That said, if you only have one brake, it's better to have it on the front than the rear.
But then, if I'm not going very fast, I only use my rear brake. That might explain it.
That just means that you brake with your rear the most. I'm talking about maximizing braking potential.
I've been riding motorcycles for decades and it's drilled into you to practice panic stops regularly, which habituates the transition from 50:50 braking to 90+% front brake. I don't think many cyclists do this exercise, but they'd probably benefit from it.
However, I like to have both (and suggest you do too!) because:
1. Brakes can fail
2. In slippery and inconsistent conditions, locking up the rear is preferable to the front
I have literally seen people flip over their handle bars by solely using their front brake going downhill.
But if your bike doesn't have two brakes as you have stated, it doesn't matter how many KMs you ride, because you are not getting that experience to inform you.
My rear brake pads last about 5000km before they need a change, my front brake pads last about 3500km.
It would appear by observation of the consumables that the front brakes do the most work. This is in-line with my expectations from the experience riding the bike and established knowledge about how weight transfer works in vehicle braking - there is a reason cars always have bigger discs on the front and some cheap hatches still have drums on the rear.