Training my young dogs I would always train them on the lead, it can be really hard work but you have no hope trying to train them off the lead.
I did a lot in my garden where it was safe & enclosed, you can do your stays/waits/recalls and then we went to a training club every Sunday which socialised the dogs but still in a controlled manner.
I don't want to sound like a control freak, it's not about that, it's about keeping my dogs safe from everything else out there. The last thing I want is to be out somewhere, and an idiot owner with a completely untrained dog tears the faces off my babies.
Well I must say with greyhounds and other sighthounds IMVHO you can never 'guarantee' they will come back, as there is always the chance that event the best trained dog will see something and take off. Even my younger whippet (who was a complete robot, instant recall) had her moments.
I let Islay off after about two months, in a field at the livery yard - I chose it because it was as far away from roads as possible, and had no other distractions like other dogs etc.
It is true what the rescue people say - when I have let all mine off the lead for the first time they have had an initial burst of energy while they run a huge circle, then they come back to you. With Islay I took some doggie treats with me for the first few times and gave her a huge fuss every time she came back, because she is very fast and scatty so I didnt think her recall would be much good. She is OK now and comes when I call, but the lure of chasing a rabbit/bird/dog/horse will always be stronger than coming to call for her and indeed for all the other sighthounds I have ever had.
The knack is to keep your eyes peeled and call the dog before it sees whatever you have seen