Building a base isn’t solely about aerobic development. Think about the #1 reason why people get hurt (besides maybe just flat out over training)... the answer:
“Quick Changes to training”
If you do nothing but easy running for a few months you are not preparing yourself at all for workouts. Now you throw 2 full workouts in per week and you’ll probably get hurt. You could maybe do 2 months of just easy running, but then you’d need at least another 4 weeks or so to slowly introduce faster running and full workouts.
Never just run easy for more than a couple weeks unless you have some sort of injury or just want to stay healthy but aren’t really training.
First, add in some strides. Super easy. Heck, start with just 2 moderate strides after runs and build up to 6 strides every other day.
Also, practice fast finishes. This isn’t necessarily a workout, but it allows you to run a little faster. On days you feel good, let it rip the last mile or 2. That last mile may be around your threshold pace if you’re fairly good, or maybe 5k-10k pace for average runners. You don’t need to plan these days, just go off of feel. I’d get into decent fitness over summer just by closing a lot of my runs sub 5:00 pace.
Add in some light workouts. For example when you’re running 25 miles per week maybe you do this workout: Warm Up 10 mins. 2x Mile at Threshold pace with 90 secs rest + 4x150m @around Mile pace with the goal of feeling smooth and relaxed.
This allows you to build your mileage because the workouts aren’t very exhausting, but keeps you used to some faster running. *You’ll also get in shape faster than if you just did easy runs.