Will I lose the distance I've spent on a walk if I decide to delete it?

No you won't lose the distance you've spent on the walk if you decide to delete it. 

We thought that would be a bit tough after you'd walked all those steps/miles/kilometers. 

So when you delete a group or solo walk, the spent distances are deleted, the distance you had banked in the first place remains. Our reasoning for this is in case your group deletes a walk but you wanted to keep your steps, or if you decided to change to a different walk then you could delete and re-spend your steps.