First up, it sounds like Brutus loves to go to the park, and his pulling on the leash is being reinforced by allowing him to get to his desired destination. So initially, I would suggest that you practice leash walking by moving in a different direction, instead of toward the park. This may mean simply taking another route in your neighborhood, or loading Brutus up in the car and taking him to a completely new and quiet location in which to practice.
Second, I’d make the act of leash walking rewarding in and of itself. To do this, you need to provide a reward – very frequently in the beginning – for walking on leash the way you want him to. Think about and decide on exactly what you expect him to do – do you want him to walk on a specific side of you? Do you want him to walk ahead of you or at your side? Do you want the leash to always be slack? Once you have the perfect leash walking picture in your mind, don’t deviate from it, and spend lots of time rewarding what you like when it happens.
Now for the rewards – aside from getting to the park, what constitutes a reward for Brutus? Hopefully you are aware of many things he enjoys, but if not, you’ll have to experiment a bit. With all dogs, food is a primary reinforcer, and is one of the easiest things to use when training. So, you need to experiment with a variety of tasty treats to come up with one or more that Brutus would be eager to eat, despite the distractions of the environment.
You might try mozzarella cheese or some chicken hot dogs to begin with. The treats will be much more interesting to him if they are tantalizingly smelly. Cut these into very small pieces, as you will be offering them frequently. Your verbal praise, toy or hand play are also good rewards, but are considered secondary reinforcers.
Be consistent with your information – place a knot in your leash where you will hold it every time, so Brutus has the same amount of slack. I’m assuming here that Brutus is a large dog, so make sure you walk with a bit of speed. In the beginning, practice in an area devoid of distractions, including the interesting lawns and shrubbery that you normally encounter on the sidewalk. Walking in the middle of a quiet street is a good place to start.
An important part of explaining to Brutus what you want him to do is accurately marking and rewarding the correct behavior. I use and suggest a clicker for this. The click sound is unique and precise, so it will be infinitely better than using your voice in the beginning. You will click when Brutus is doing exactly what you want, and then follow the click up with a treat. The goal here is to make leash walking itself an rewarding experience for the dog.
From your very first step – out the front door or out of the car – you will very frequently (like every two-to-four steps kind of frequently), click and reward Brutus when he’s walking the way you like. Anytime he starts to pull, you have a couple of choices – either abruptly stop moving, or abruptly turn into him and change directions. This is where the consistency pays off – through some repetition, Brutus will realize himself that walking on a loose leash yields more enjoyable results – he gets goodies and praise from you, and he gets to keep walking forward. The undesirable behavior – pulling – will fade away, as it is no longer in his own best interest to do so.
Once you have built some good leash walking skills, you can begin to walk toward the park once again. Just be consistent with your information, and make sure that each time Brutus begins to pull toward the park, you stop or change directions. When you resume walking, click and treat along the way.
Finally, vary the times and locations at the park where you will release Brutus to run (and do so safely and legally, with either a long line or retractable lead attached, or enclosed in a gated area meant for dogs). Sometimes you will continue to expect and reward Brutus for walking politely on leash through the park. Other times, you can choose to release him to run almost immediately. The goal is for him to never know exactly when he will be offered the reward of freedom, and for you to continue to reward polite leash walking, and not reinforce pulling by letting him go.