Karen, with only a few years experience at this I'm by far not qualified to give great advice! Peter's advice on taking it slowly is good.
As you mention she is running passed you, you may want to consider standing at a location where Mazy can't run passed you. I have found standing in locations like the 1.5m wide pathway at the side of my house with a gate at the end helped to draw the dogs towards me and eliminated a lot of the opportunities for them to get distracted on the way back with the dummy. Once it's working in the garden then progress to other locations but again stand in places where she can't run passed you eg up against fences. Also you crouching down for the dummy can help.
I never got on well with a long lead. I just got in a mess with it, and ended up flat on my face and winded too many times!
Another bit of advice is listen to experienced trainers and be careful with advice given from beginners like me :) So if Peter cautions you on any of what I've written listen to him and not me!
I hope you are enjoying the training.

Well done to Sandra and Dee with their bronze KC. I've not heard of starters obedience before; what do you have to do for that?