It's a two and a half hours/hour's walk.

I'm not sure I'd use an apostrophe, because the hours don't possess the walk

I disagree with the previous posters. British usage is a little different.

I would say two and a half hours walk, and I might write it with an apostrophe, thus:

To get there requires a two and a half hours' walk.

That is, a walk of two and a half hours. The apostrophe makes sense to me.

cf. a man's suit, the day's ending, the tasks' duration.