how does the dog walking work while I'm away from home? Do i leave a key?

That's up to you and should be discussed with the dog walker at your Meet & Greet. There are a number of options, including:

1. You can give them a key or leave the key hidden somewhere on your property.
2. You can get yourself a lockbox and give the walker the code to retrieve the spare key inside, which would be returned each time.
3. If you have a garage, you can give the walker your garage code.

The walker may have other ideas. Just discuss it with hm/her but that is generally how it is done.