I just replaced all 4 wheelsets with new pads and wear sensors. At the last service the dealer said I would be due for brakes at the next stop. Knowing what the dealer charges, I elected to do it myself. At 39,300 miles they where about 1/2 worn. I can't say that I noticed any change in the brake action as they wore but, I did notice a firmer pedal after the change. I guess due to that much less fluid in the system. I put a disproprotionate number of highway miles on the LR3 and maybe could have gone another 20,000. If your at 40,000 miles and your going to do the job your self, don't wait. Do it now and while you're at it throroughly clean the rims, put anti-seize on the hubs, studs, and lugs, and get new tires. The improved ride is worth the money spent.
As to changing the brakes, I had to use a narrow 15mm (???) spanner to keep the posts from turning while I removed the caliper bolts. It helps if you have a bike bearing wrench as these are really thin.
I replaced the wretched Goodyears with 5 Michelin 4x4 Diamaris 275/55/19's. This slightly larger size put the spedo dead on instead of reading about 2 1/2% higher. This is the largest tire that will fit the spare well while inflated. The ride is unbelievably better.