How old is your dog?
You can use that 50 foot lead to train a good recall aka the "Come" command. Over and over and over. Then when she learns that, you can switch to learning it off leash in your yard or other fenced area. Then you can think about letting her off leash without a trailing line.
My dog was at least 2 years old before he learned to come when called, reliably. Granted, I'm not exactly the obedience master
I am sure that if you were dedicated, and/or took some classes, you could teach it earlier.
At 4 years old I was able to teach "Wait" which to me means 'stop until i get there' and used that to keep my dog from stepping off curbs.
I am sure many people who are actually good at obedience training will disagree with me... but it always seemed unreasonable to me to expect a dog to learn something that a human wouldn't be able to learn at the same age. I mean, they're not as smart as we are, so how are they going to learn, at 1 year old, not to chase a car? Except the hard way, and nobody wants their dog to get hit by a car!
I want to add one thing - I had a friend who was in the habit of letting her dog run with a 50 foot lead. It became an issue because it would get caught on things and the dog learned that if she yanked really hard on the lead, it would come loose again. Not really a great lesson to learn for leash training imho. Plus one time the dog jumped in a lake and started swimming with the long lead attached and it got caught on dead wood that was underwater. scary. It sounds like you are supervising it closely, so it shouldn't be a problem for you.