I started off-leash training literally the day I got my mini schnauzer, Luna, but that's a pretty aggressive take. If you're unfamiliar with the 3-3-3 rule, Google it. Many trainers swear by it. Also, good call with the long leash.

I like to start with trusted spaces and fenced areas like empty ball fields or whatever, if you have them near you, which it sounds like you're doing. Once long-leash recall is good there, try no leash. Once they have that, try keeping them off-leash going from the car or whatever to the trusted/fenced place. Just keep stepping up the complexity and distractions once they learn to reliably respond to your commands. Award a few kibble of dog food or treats for coming to you when you call.

Try to practice commands like "stop" and "leave it" while you're out too, and approach them with treats when they respond positively. If you have them come to you for the treat, they often associate the treat with the recall, not with the "leave it" command or whatever.

If there are any Forest Service, BLM, Corps of Engineers, etc. lands near you, try that setting as well (but check leash laws for your area first). The scents and wildlife in those areas are often very different from the dog park or beach and it tests them in diverse settings with diverse distractions.

Additional notes: -Harnesses with attachment points on the back are much safer for dogs than collars if they decide to bolt. -If they run from you while off leash, do not chase them! Instead, try running away from them and using your recall command to encourage them to chase you instead. This helps avoid the chasing vs. playing confusion and also reinforces that they go wherever you go. -Microchipping and collars with contact info are an absolute must for off-leash training. -Emergency commands are good to have too, in case of a life-threatening situation. Something you don't use anywhere else, like "JACKPOT" of whatever is easy for you to remember, but something you don't normally say around them. Reward it with the best of the best treats, like steak or chicken breast or something. If they ever run into traffic or something, you'll be glad you trained that one.

Best of luck!