What's Your Mileage Baseline?
To plan training that's just right and improve your running, you need to know your baseline mileage. This is the total number of miles that you're comfortable with every week that still leaves you feeling recovered and not at risk for an injury.
Look back over the last six months in your running log. Try to find a range of mileage where you felt the best. If you don't keep a running log, you'll need to guess (sidenote: start writing your workouts down!).
This is your baseline. But to continue improving, you need to train over your baseline to create more fitness adaptations.
The best way to do this is to add a mile to your long runs every two weeks. You can also increase your total weekly mileage by 5 to 10 percent every 1 to 2 weeks once you're over your baseline mileage (you can be more aggressive when you're under your baseline).
It's best to make mileage increases every two weeks instead of every single week. I call these "adaptation weeks" that help your body adjust to the increased workload so you're not doing this extra volume too soon.
Cycle Your Training
Of course, you can't be above your mileage baseline all the time. Sometimes, you need to take some time off to rest and recover. This is called cycling your training by alternating easy (under baseline), moderate (baseline), and hard (over baseline) running over time.
You'll see that most training plans are set up in a similar fashion: the first few weeks are introductory and should be beneath your baseline mileage. Then there's the phase of running higher volume above your baseline, an easy taper, and then a recovery phase after your race (the same is true for cycling strength workouts).
This process is repeated and, over time, your baseline mileage will gradually increase so you can do more volume and faster workouts at the same effort level with no additional risk of injury.
The big picture here is that the goal of a training cycle is to push yourself briefly beyond your baseline to produce fitness adaptations. And the goal of a few training cycles is to increase that baseline so you can continue getting faster.
More: How to Train for a PRSign up for your next race.