Hi
You are already making progress, so you know that what you are doing is working!

YOU have to be the consistant one when Roxy has an 'off' day and tries all the tricks in the book to test your patience! Having got so far, do NOT give up now..........Roxy has learnt that pulling brings its own reward in the past, so it is going to take a long time of being 100% consistant to undo the months/years of successful pulling. You are not teaching a 12 week old pup who has a blank canvas and never been on a leash before, so it will be harder and it will take longer. Your resolve to sort this problem has to be absolute!
You have discovered the pitfalls of using food as a bribe; by all means use the titbit to lure her back to your side but then don't deliver the treat until she has walked a couple of steps on a loose leash. Then you build up, adding a couple of steps at a time. Alternatively, if you don't want to use food the walk itself has to become the reward.....you use the same principles, dog pulls, you stop, wait for the dog to decide to come back to you and focus on you before moving forwards (the reward).
Also, you may want to consider using the gentle leader (your management tool) when you need to get from A to B in a set period of time - ie to go to the park to give her a free run. Then, set aside some extra time in the day for the loose lead walking....then it doesn't matter one hoot whether you get 2 yards, 20 yards or 200 yards! :-) Also, two or three 5 mins sessions a day may be more beneficial at first rather than one 15 mins one.
Good luck and keep persevering! Your'e doing great!