For a lot of runners, especially newer runners, the mistake is going hard all the time. When I first starting to run regularly, my thought process was that if I ran hard all the time, on race day my hard pace would feel easy. That seems logical enough if you don't understand that different types of runs have different sorts of physiological benefits. Now that I've been around the block a few times, I've come to realize that if I go too hard on my easy days, it really screws up my hard training sessions. I notice that the most on Tuesdays, which is my intervals workouts day. If I push the pace on my long Sunday run, I have a really hard time hitting my targetted paces on Tuesday, so I make sure I watch the pace. I generally try to keep my long run pace about 30 seconds/ km slower than my marathon pace. One note on that- I often do the last couple of km at close to marathon pace to get my legs used to finishing fast, but overall, I aim to keep my long run pace fairly comfortable. If I do that, I generally do pretty well with my Tuesday intervals. That's the "Train Slow to Train Fast" bit of the blog title.
The next bit is "Train Fast to Race Fast." As important as it is to keep portions on your training slow, it's equally important to do some of your training at the pace you plan on racing. In my current training, I have 2 speed sessions per week. On Tuesdays I do intervals, which, depending on the distance, are run at either 5km race pace or 10km race pace. It's really helpful to do those sessions where you legs get used to turning over quickly. On Thursdays I do my marathon pace sessions, which are generally longer- early in the training cycle I might do 12km with 8km at marathon pace, and as it gets closer to race day I'll do 20km with 16km at marathon pace. Getting your legs used to running at the pace you plan on racing at is really important, and it's amazing how they'll catch on. The first marathon I ran after incorporating the long marathon pace sessions was Philadelphia 2012. In all my previous marathons I'd hit the wall pretty hard around 32km, but in that one my legs just kept on turning over at the pace they were trained at, and I had a great race. It's pretty far fetched to hope that race day magic will make your legs move at a pace they've not trained at.
Training slow and training fast are both key to racing well. Figure out how to balance they two and you'll set yourself up for success in your next race.