Since posting my Tough Mudder training video the most commonly asked question usually is what type of training advice can you give me? Where should I start and what do you suggest? The response I've given everyone is what my martial arts instructor always used to say... "Whatever works for you."
I'll try and pin it down a little better for you though. Most endurance races, mud runs, half to ultra marathons and all levels of triathlons can break you down mentally even when your body has more than sufficient power for the task at hand. It's like when people are at the gym and they stop at too few reps when they know they could bang out at least three more. "Mind-Body Connection" is really the key here I think ("Mind-Body Prescription", GREAT book by Dr. John Sarno). One of my old martial art teachers used to say that if you enter a competition with the mindset of "Well, I'm just going to do my best, and if I lose, no big deal." then you might as well step off the mat because you're a loser. Harsh words but it started to make sense. Needless to say that every race, competition, workout or sparring session I've entered into since then I've had the mindset of "Today I will dominate." Now sometimes I don't and I flat out do horribly but then I say, "Next time I will have my revenge." Now trust me, I'm not some crazy Fitness Nazi and I don't say these things out loud but the mindset I have is just that. Plus, with my age getting up there, I think anything less and you are not going to see results I think you are looking for. Does that make sense? Hell, I've read stories where a person who has never run more than a 5K decided that they would run an Ultra Marathon and go literally from couch to 50K. There's a British comedian named Eddie Izzard who decided to run an insane amount of back-to-back marathons (I think it was something like 40) for charity after never really running in his life. Sure, you will feel more pain after than the person who has trained but it is entirely possible. So to sum it up, the game is 90% mental.
The physical training I do is what works for me. Plus I am constantly switching things up. The general key I use is if I find a particular exercise where I feel weak or it's hard for me to do with good form then that's the one I focus on. Not to get too philosophical but Bruce Lee used to say "To physically express oneself honestly is a very difficult thing to do.". It took me awhile to wrap my head around that one. I knew it was easy for me to do certain exercises and harder to do others. Why didn't I do the one's that were hard? Was it my ego? Was I afraid of looking silly? What would happen if I focused on those things and actually became proficient at them? Breakthrough bro. I suggest trying it. If you are a distance runner try hill sprints or weights. If you are a weight lifter try running 5 or 10K. Your overall health will grow exponentially.
Hope that helps in some small way. Best of luck and never stop.