My very favorite book when it comes to the mental side of athletics/endurance is "Do Hard Things" by Steve Magness. It is a practical and tangible guide to the mental conditioning aspect of training. He is also really well balanced when it comes to life, managing priorities, staying in community, etc. I like Goggins and Jocko and such, but the idea of "just grit up and get it done" isn't sustainable to me. Magness presents a framework that is grounded in vulnerability and sustainability. That is a more macro view I really like.
A more micro approach that I like is ACT, which stands for Acceptance Commitment Therapy. ACT is a psychology theory and practice, but it applies well outside of clinical practice. The basic summary is to accept things that happen to us (good and bad and easy and hard, but particularly bad and hard) and reframe them by committing to something we value. Thinking, talking, and acting from that mindset consistently has changed my running and positively impacted other parts of life. "Do Hard Things" includes detail and application in ACT, among other approaches.
Quick example is "this run is really hard at mile 16, and I'm struggling". ACT would tell me to accept that it's hard (it is) and accept I'm struggling (I am) but to reframe my approach to something that is intrinsically valuable to me (it's my longest training run yet, I'm part of a team effort with my daughter, there is more to this training program than just this mile, etc). As opposed to trying to change reality by telling myself "this isn't that hard" or something like that which causes distortion between perception and reality, I am changing what I am going to do about the reality.
Another thing I do is intentionally set my laps and alerts on my watch to 5k and 10k increments for long runs and races. I don't need or want a good/bad evaluation every mile and if my watch splits and buzzes, I am going to look. And if I look, I am going to evaluate. I'll even go as far as setting really weird lap increments to skip over miles or sections that I struggle with. I used to have a huge wall at mile 17 (like every training run), and I would intentionally skip the increments at 25k so that I went from 20k to 30k and tried to not even pay attention to my watch between them. I didn't want to know mile 17 was happening until it was over was the idea.
The long runs as it warms up are definitely tough. The couple of things that help me is to adjust pace as needed to make sure and get the mileage in. There are going to likely be days that are 20 to 30 seconds per mile slower than right now. It's just part of it. So, disconnect expectations from pace and attach them to completing the distance. I personally believe that running a prescribed 18 miles slower than "normal" is much more beneficial than blowing up after 14 miles due to running "faster". The other thing is to really up electrolytes during warmer runs. I use Salt Stick pills and chews liberally, and they really help. Most gels and the like don't have electrolytes, and I can't do all the sugar in sports drinks.
This is terribly long, but I really do love this stuff and can get rolling when I start. I hope that a piece or some of it helps in some way.
