As others said let the pup off the lead asap. If you wait till she is any older she will be far too independent. If you wait till she is a year she will never have learned to come back and will never be controllable - never heard anything like that before. Go for walks where she cant come to any harm if she wanders off a little way and let her off for short spells then on a flexi lead so you can reinforce the recall. I cant quite see why an extendable lead should teach a dog to pull. Surely you have the dog walking beside you until you release it and allow it the full length, in exactly the same way that it walks beside you until your release it for a free run.