Go to obiedience classes to teach your dog general obiedience including a word Come.
The problem is that whatever the world offers is obviusly more iteresting to the dog than you are, hence the dog goes exploring. If the dog would know that you are fun, love, food etc any time would stay closer to you.
So in order to achieve this you have to develop a bond with your dog. Make a "monkey" out of yourself play with it, jump around, run screeming and waving your hands and if your dog follows you than reward. Something very tasty, something that is only given on these extreme fun outside little trips to the park. If you just walk along how interesting are you???? where the grass smells of other dogs, sheep etc. Would you be more interesting if you had food and gave it to your dog for staying close? or if you had a favourite toy and was willing to play with it, or something to fetch? and if you were on top of this sooooo happy to do it! You bet!
Teaching your dog to say close is a long process, especially with a grown up. But it can be achieved. Your dog should be relying on ou for reasurance, so in new places automatically should stay closer, if he is not ask yourself who is the boss??
I have tought my dog by teaching the word come for starters, than on the long lead the same thing, I used a dog training classes for this, distructions of other dogs and people.
Secondly I have a release comand and I agree for my dog to go and run around free and play with others when I agree to it. After all they need to do what dogs do, not only what I say all the time.
I also used to hide. If my dog went to far away I called ONCE - come if he didn't I dissapeared. Countless times I climbed a tree, hid in the bushes, behind buildings etc. Of course you have to choose an area where you ca actually do this sort of stuff, and know that nothing will happened to your dog. There were times that I was in hiding for 10-15 minutes and watched my dog run around looking for me in panic. Believe me its a long time. And when he found me - I was over the moon, happy, playfull and full of treats to give him.
I have also driven off in my car few times. Same scenario, I called the dog ONCE, didn't come, so I got in the car and drove off. You should see the speed he followed!!!
Now I am like this old lady that you mentioned My dog walks, runs and plays with others, but looks up every so often to see where I am. When alone walking in the park or in the bush I really find it hard to hide these days.
My outings with my dog certainly do not look like I am walking along bored, checking time if its time to go back home, and the dog does what he wants (runs away exploring what he wants). We play, we jump, we have 3-5 minute training sessions, followed by more play, rolling it the grass, healing, fetch etc. I came back worn out (I have a large dog) but boy is it fun - yeah, we both love it.
Again - the most important is that you are more interesting to the dog than the rest of the world, how you achieve this? you know best what your dog likes most so provide it.
It is a long process, good luck do not give up after 6 months, it can take longer with a grown up.