LOL Mud's reaction the first time I used an extenda-leash on her was funny. It was actually our Chihuahua's; I am sooo against extenda-leashes because they are so often misused, but I put it on her when we were out of town; don't remember why. I gave her her release word that I use to release her on on-leash walks and she looked at me like, "WHAT?!?!?! I can't run off when I'm on-leash! That's not okay Mom, what are you thinking? Have you gone mad?" It took a little bit of riling her up to get her to kind of walk off in front of me a ways, but she wouldn't go very far on it. On leash means I can't walk ahead of you Mom, don't you remember?
Nathan...
good advice already. Recall is an absolute must before working on off-leash training. There is a lesson in the classroom on recall. Work on recall a LOOOOOT, and work up to your dog being able to consistently come to you even with lots of distractions. When you start off-leash training, you want to start with a long leash(30-50 feet or so) and let them drag it. This way if they do try to run off, you can keep them out of a sticky situation by picking up the leash.
Don't start off-leash training until your dog can walk properly on-leash. If your dog can't walk next to you or very very near you on-leash, testing off-leash is just a terrible idea.
I live in the country, and many of my dogs' walks are out in a field. 100% of these walks are off-leash. But, it is very structured. My dogs have to stay in heel position for part of the walk. After we've walked a little ways and they are paying attention and behaving, I release them(I have a release word that I use to let them know that they don't have to be glued to my side anymore) and they are free to run. But, they can't go out of earshot. So when they get not quite out of hearing distance, I'll call them back. They come back, they have to get back in heel position for a while, and later they'll get released again. In a 1 hour walk they may get released 10 times. Depends on their behavior. If they aren't doing so well, they spend more time walking in heel than running around. They are rewarded for walking nicely by getting to run around as they wish. But, they have to come back. Understand that this works for me because I live where it is safe for me to do this, and my dogs have very good recall. Mudflap has the best recall of the bunch; so far (KNOCK ON WOOD!) I have not found any situation that I cannot call her away from. But I have done a ton of work with really tough distractions to get her to this point. It's all about baby steps. Zeke is not this trustworthy yet, so most of the time his "release" is still on-leash, but on a 100 foot leash. He can run just about anywhere he wants, but I can keep him safe. Gypsy is my newest, and I am already starting this with her. She walks nicely, and she gets "released"--30 foot leash attached. Gypsy does not get to run around for very long at all; she is still very early in her training and you don't want to lose their focus by letting them have too much time to sniff around. She gets less than a minute of free time each time I release her. Nonetheless, it's still rewarding for her. Z gets about 1 minute at a time, and Mud can get up to 2 minutes as long as she's checking in with me. She could probably get more, but you don't want to give them too much time. Too many things could happen, even though I fully trust Mud's recall. The more time they have, the further away they can get...you're that much further away if anything happened and you needed to get to them quickly.
"CHECKING IN"--Mud has learned that when she gets a certain distance away, I'll be calling her back to make sure she's not too far away. So she'll get to a certain distance, stop, and wait for me. If I don't call her back, she'll just wait till I get closer, then run off again. So she "checks in" to make sure she's not too far away since she knows I will be calling her back.
Gypsy eventually will be my hunting buddy and will be working a lot off-leash, so I'm spending an enormous amount of time on her recall. Start somewhere low-traffic, where people and dogs are visible, but it's not overwhelming and you are a good distance away so your dog can still focus on you. SOME parks can be a good place to try this. Your dog is on-leash, so it's safe, there
are distractions, but fairly far away. Work on recall here after he can reliably come to you when called at home with very little distraction. Then maybe try to get a little closer to the traffic(traffic meaning people and dogs in the park, NOT cars). Work your way closer and closer(over weeks or even months depending on your dog) until your dog will come to you even with joggers passing him, dogs walking past, etc. This is a big distraction for your dog and it will take you a while to get here.
Hope this helps.

Good luck and be safe.