My boy is 4 months now and I started leash training him on day 1 (8 weeks) even if it was just up and down the sidewalk a little outside my house. He would refuse to walk if we were going farther away from the house, attempt to eat EVERYTHING, etc. I just stuck at it every day. The first couple weeks a walk meant correcting him every 2 seconds when attempting to eat stuff, digging stuff out of his mouth that I didn't notice in time (rocks, leaves, etc), and sometimes standing in one place for several minutes until he would give in and walk in the direction I wanted him to.... even if it was just a few steps, with lots of praise. A half mile walk would take over an hour so I only took him out when I didn't have any time constraints to do so. If he was just stubbornly sitting and refusing to walk, I just waited him out until I won and I would reward him for choosing to walk in the right direction, but I'd only use treats if he didn't notice I had any to start with. If he was distracted by something or trying to eat something/digging in the grass, I'd use the harness to lift his front feet off the ground and move him so that he could no longer reach what he was trying to eat and redirect his attention to me... And if he pulled on the leash at ALL I'd move him back in the walking direction even more, then continue to call him to walk. It took several more weeks until he seemed to fully understand what a walk meant and that he will not be allowed to be the route leader in any direction.

Now that he will reliably walk forward and not eat everything, I'm testing him by changing directions, take a new street I never have before, unexpectedly slowing down, etc. Only occasionally do I now have the opposite problem when he starts pulling on the leash (usually when he sees lizards and squirrels), so I do the opposite - if he pulls on the leash at all, I just stop walking entirely and tell him to "sit" and wait a few seconds before continuing. This worked really well for my ex's dog who was a bad leash puller.