
Long line would be best, yes - that way if she ignores you, you can stand on it (don't pick it up, or she'll associate you with it and when it's gone, she'll ignore you again!!) and walk along it to her, then bring her back to where you started. Here's the method I'm using on my 2 yr old and very independant pair of dogs:
http://www.shirleychong.com/keepers/Lesson6.html
I haven't really stuck to the week-per-step thing, more just worked with each of my dogs at their particular level - for example, my dobe is ready for the long line now, but my rott X is not - still too distracted. So she's still on the indoor distractions.
I'd make sure she always responds to you without ditractions before you try the whistle with them - if she ignores you at any point and gets away with it, it could knock your training back. It's a problem I've had!! You sound like you're doing well though - when she's quite reliable, I would make a list of places you can take her to generalise the whistle (that is, make her respons wherever she is - dogs aren't good at mentally transposing (right word??) commands from one situation to another by themselves, you have to train the same thing in different locations. If you can find a spot liked by dog walkers, so much the better - even better if you can find several with different numbers of dogs.
I go to my nearest park for low-distraction training (the odd dog, and we know most of them - strange dogs are more interesting to my two), the next nearest for higher distraction (more strange dogs, squirrels), and we will be going to a beach called Botany bay for the final training - my dad informs me it is packed with dogs on a nice day :)
Make sure she has a strong association between the whistle and whatever reward you use - also you may have to up the reward in a more distracting place. I can use cheerios indoors, but I have to use tinned ham outside.
Good luck!