Don't worry too much about teaching them to lead. It's comes pretty easy to horses. Usually the biggest problem I have is the horse I'm riding getting used to a lead rope getting under his tail or slapping his flanks. But with a little practice they learn to deal with it.
Also some horses get a little excited about having the pack horse right on their butt. They may want to kick. So I have to teach them to be tolerate of the second animal being nose to tail.
My best advice it to use a long lead rope and just hold it in your hand. If the pack horse, refuses to cross something or pulls back, You can let out some slack or even just drop the lead. I find my horses being herd animals stay close even if I drop the lead, So it's easy to go back and pick up the dropped lead. But never tie the lead to your saddle where it won't pull free.
I think its great experience for young horse to follow along on the trail. With no weight on them, They are not stressed. But they learn to deal with stirrups flapping, lead ropes flopping against them. They also learn to cross creeks, logs rocks and other trail obsticles. If they do this as a 3 yo, Then as a 4 yo when you start to ride them, These things are old hat and they just need to learn to deal with balancing a riders weight, and the various cues the rider is giving. Just keep them to shorter rides. 10 miles is plenty for a 3 yo. Wild Mustangs often cover 15-20 miles a day for water, The young stock follows the herd across somre pretty wild terrain. That's how they learn to traverse rough country in the wild.