Couldn’t agree more, especially with (1). After running my first marathon a few weeks ago (3:46) I’m convinced more than ever that it’s hugely a mental game. My longest run had been 16M (not intentionally; an earlier injury was a huge setback) and my weekly mileage was pretty bleak (never more than 35M). I was close to calling it off, but was too late to defer and decided to go for it. 
1. If it’s your first, start off conservatively (if it’s not your first, you know your ability and better). Starting off slower than you want/can is very difficult for me, but paid off at the end!

2. I was dreading “the wall”. Once I ran past Mile 18, all I could think of was “the wall will come now”. Well the wall never came. Mile 20 came, and that’s where I did a mental reset of “hey, it’s another 10K to go, you can always do a fast 10K”, so my 10K was in fact my fastest split by far. I learned the “mental game” at yoga and hills running - strongly recommend practicing it on your runs and other activities. Mental reset for the past few miles does wonders!
3. It’s ok not to follow a training plan exactly. Don’t get discouraged by it. Focus on getting one long run and total desired mileage and see what training plans works best for you (some suggest your longest run being 16M but insane weekly mileage, others recommend longer runs). Ah and make sure that your easy runs are in fact easy... I got a serious hip injury by going too heavy on my easy runs...