Are all the sentences correct?

All correct.

1. I walked five miles a day - I walked five miles a day at some time in the past. The simple past is used to talk about a completed action in a time before now. Duration is not important. The time of the action can be in the recent past or the distant past.

2. I was walking five miles a day - at a certain time in the past, until something happened, e.g. I got a car, I changed jobs, I broke a leg. The past continuous expresses an unfinished or incomplete action in the past.

3. I used to walk five miles a day. I don't walk five miles a day any more. "Used to" expresses the idea that something was an old habit that stopped in the past. It indicates that something was often repeated in the past, but it is not usually done now.

Thanks, contrex. I appreciate very much your detailed and clear reply.

She's pretty good isn't she Tang

Who is "she"?

I thought you, Con; but evidently I'd disremembered. It's oft tough w/no pro

Here I am in 1978

Yea Con but which one is you

Just kidding Con, with profound apol

In that picture you look like Malcolm McDowell in Time After Time!

Yes, I know, it was spooky at the time.