How to Write a Long-Range Series

I think it just comes down to being observant. And also constantly being on the lookout for what's going on with me. How can I use that? When I wrote the second book in my Eli Mark series, I was at the time kind of out of the blue experiencing these weird panic attacks having to do with being in a situation of height. Don't know where it came from, glad it went away. But I absolutely used that because it was something that I could write about effectively and realistically. All of a sudden he has to deal with this along with everything else.