Harness is a good idea.

I think most people will trust you..

But so they know you're serious and that they know they can trust you, you should question them, and ask to meet the dog before you walk.

Ask them the dislikes and likes of the dog, ask if he is good with children, other dogs.. that way you avoid hazards. Meeting the dog allows you and the dog to get to know each other.. which is always a good idea in case he gets away on you on your first walk. (Not saying he will, but I said the same thing to myself!) not a lot of dogs would like to come back to a person they have no liking for.