present perfect simple vs. present perfect continuous

I'm tired. We (a)   (walk) for over an hour. Let's stop and rest for a while.

The zoo isn’t far from here. I (a)   (walk) there many times.

The telephone (a)   (ring) for almost a minute. Why doesn’t someone answer it?

She is 80 and she (a)   (never / read) a book in her life.

Sally is writing a letter to her boyfriend. She (a)   (write) it since she got home from class. It’s going to be a long letter.

I (a)   (write) my friends at least a dozen letters since I left my hometown.