Cosby was 2 year old before we could let him off the lead and I know how frustrating it can be. We tried everything, training clasees, different treats of every shape and size, waving our arms in the air, making high pitched funny noises, running in the other direction - nothing seemed to work. He took off in a straight line at great speed and if he didn't want to come back he didn't. He would do the look thing and then continue running away. He would also be straight over to every dog and person, even if they were miles away on the horizon! He even had dog's that tried to attack him, but that wouldn't stop him, he'd still run back for more.
It was really stressful and you're right, they don't get as much excersise on the lead. However, if your dog is not good at recall don't let it off the lead just because you think you should. The main factor is the dog's safety. There was more than one incident when we thought we were getting somewhere with Cosby and then he would take off and we thought we'd lost him. There was even one incident when he got distracted and rang all the way home - over 3 miles away and across the roads! Oh and not to mention the time he ran off and got stuck in the River!!
All I would say is, I know how frustrating it is, but keep at it. Use a training lead to practise when you're out and slowly she might get the hang of it. Don't be tempted to left her off in areas which she could run away and if you are going to let her off to practice go to a area that is fenced in. Keep practising with other commands and general obedience and the more she starts to take notice of you, the more she is likely to listen to you when you're out. As for excersise, while you are still training perhaps you could take her our for more walks on the lead so she is still tiring herself out. Or maybe go running with her, or play games in the garden.
I'm no expert and I don't know much about dog training, but I do know how stressful it can be. I hope she gets the hang of it soon
Emma