yes a 15 ft training lead is perfect.
the idea is that they get far enough ahead that they are intent on their sniffing that they are paying YOU no attention.
Then with the lead still loose you turn and walk in the opposite direction.
when it snaps tight she will come back to you. As she comes level praise her so she knows this POSITION is a good one.
She'll go past you ahead, repeat this process.
You will probably not go more than a few hundred yards the first time you do this.
BUT
Eventually she will realise that you keep changing direction, and will start watching you for the change.
BUT while she is watching you she's walking nicely so you keep on going.
When she loses focus you change.
She'll get the idea.