I am starting a new group walk and using the app. How can I get rid of all the miles I have clocked up so far and start fresh?

You cannot reset your steps to zero. That is not how World Walking works.

https://worldwalking.org/help/article/3801/we-are-starting-a-new-group-walk-is-there-a-way-for-our-group-members-to-delete-their-previously-recorded-steps

Our apps show just the total number of steps walked in total by each group member. However, when you start a new walk, solo or group, your steps total for that walk begins at zero. You can view this if you log into your account via the website rather that the app. The website has more functionality than our apps.

https://worldwalking.org/help/article/6175/do-you-have-a-leader-board-which-will-shows-how-many-steps-each-group-member-has-walked