As I train and run the longer distances I am struggling with my thoughts and my head.

I have focused on the physical comfort in the run- now need to work the mental aspect.   Not a fan of No Pain No Gain but I do know that some "stress" is needed to improve or even to maintain as I age.

Any books, or other help for this?

There is a great book called "Running Within", sorry can't remember the author right now.  Francesca and I just had Anne Riddle Lundblad speak at our store and she swore by this book.  Anne believes the techniques in this book have greatly helped her performance.

Anne is the USA Silver medalist runner at the 100k road distance!

Running Within    by Jerry Lynch and Warren Scott.1999

Published by Human kinetics http://www.humankinetics.com and available at Amazon

Lots of great tools to improve the mental aspects of our runs and can be used for other life issus.

As a lifelong athlete I have seen the value of Visualizations(picturing my performance) and Affirmations ( self-talk in a positive present tense mode that prepare me for success)

With "my body" I can visualize and affirm winning the Kentucky Derby as a jockey but since I am kin to the horses ( and kinda slow!) it is not very realistic.   Visualizing and Affirming running and completing a 26 mile race is within the realm of possibility.  Winning the race..... Not really.   I have read the book and applied the  ideas to my workouts and life.