He’s an old dog, and he’s set in his ways. If he’s a typical terrier, he must like food… if he does, use this to your advantage. You can certainly use the harness. Lure him with some grub for handling his legs, and try to work as fast as you can (or have a helper). If you think he’d do better with it, you can also try a head halter. They work much like a horse halter in that you end up controlling where the head goes. Really useful when you want to make their heads come up (stop sniffing, darn it!) and they can’t really pull from their noses!
The key to making him stop sniffing/pulling and actually have him HEELING so you can walk is to make YOU more interesting than the rest of stuff out there. If he’s food motivated, go about it that route. Show him the GOOD STUFF (hot dog bits, cheese, good high-value treats) and hold it up against your chest, start walking. He will quickly learn to look up at you while you walk. Praise him (if you clicker train, I’d say click, treat). Reward him for doing exactly what you want him to - heeling, looking up at you, not pulling, etc. At first, this will take time… but in the long run, it’s well worth it. You can also teach him the “look at me!” command so you can get his attention back when he’s busy sniffing or thinking about marking everything along the way. Again, he’ll initially only do it if YOU are more interesting that “IT”. It’s tougher with an old dog that’s set in its ways, but if you keep sessions initially short and fun, he’ll catch on in no time.
Principle of Operant Conditioning: They engage in behaviors because they get something out of it.
So your boy gets something out of the pulling, marking, and being a yutz while you try to walk. The idea here is to change the equation, make it WAY more valuable for him to pay attention to YOU, heel properly and enjoy the walk… Find his currency and go with it.
I have two VERY food motivated dogs. One is not, and is toy motivated. I can make him do ANYTHING for his rolled leather tug toy.