Your event, be it 26.2 or 3.1 miles, doesn't really care what you did the week before the race.
It doesn't even worry too much about the two months before that.
What is is concerned with is your frequency, consistency, and volume of miles over the long term.
We're talking months to years.
Every day adds up, a marathon is worth more than the sum of its parts.
Every little decision matters, and the consistency of positive choices makes a difference when you get to the starting line.
Training is not only the time you spend on the run, but it is the decisions (positive or negative) that you make along the way. Below is a list of common negative decisions that all runners should be mindful to avoid.
Running too hard on easy days
Pushing the pace when running fast matters the least is a common error. It's tough to run slow. Yet, there is almost no pace too slow for your recovery or easy base building runs. A great (and enjoyable) method of keeping the pace under control is to run with a slower training partner.
That being said, if you are feeling a bit perky that day, go ahead and run the last quarter or 10% of the run at a more moderate effort. Much better than doing the entire run at a slightly too fast of a pace, doing the last mile or so at a good effort will not affect your recovery, but still gave you a bit quality. A nice guide is to keep easy runs within 1.25-1.5 times your 5k pace.
Judging a workout by how the first mile feels.
Ignore it, always. The first mile is a liar. This is especially true in the mornings when you're often on the road as soon as possible after waking, so you've not given your body time to warm up (or had your coffee!)
Making up for a missed run with adding it to the next run
If you missed a long run or track workout due to scheduling issues, it's no issue shifting the training run back a day, but do not think adding miles or intensity to the day after the missed run is beneficial. You missed that run, it's gone and there's no getting it back. Think of the day as an extra recovery day, which is rarely a bad thing.
Not being mindful of recovery
Speaking of the "r" word. Training actually makes you slower. It breaks you down and tires you out. It's during those days off and the easy recovery runs that your muscles rebuild, capillaries multiply, and your slowtwitch fibers strengthen.
Long runs that are too long
Consider this for a moment, most elite marathoners running 100+ miles a week are still only doing 20-23 mile long runs, and that's only 17-23% of their weekly mileage, yet amateur athletes are often doing the same amount of long run distance on half of the weekly mileage and less optimal recovery.
Long runs of 20% weekly volume or no more than 3x your daily volume are good benchmarks, any longer and you may be running longer than your ability to recover, which can affect the runs following the long run. Daily volume over a long period of time and being a consistent runner are far more important than going super long too often.
And remember, you do not need to run long every week! If your schedule allows, try doing a 10 or 12 day training cycle or doing a biweekly long run and a biweekly tempo run of 2/3rd the length of your long run.
If you could go back in time and give your new runner self a piece of advice, what would it be?