Your initial focus should be on more aerobic training if you want to get faster. If you haven't been running for very long, aerobic training may be the only type of training you do for a while.
Increasing Weekly Mileage
To avoid injury, increase your mileage by no more than one mile per day per week. For example, if you currently run 20 miles in four days per week, run no more than 24 miles the next week by adding one mile to each of the four days. Don't add all four miles to only one day of running.
If you're a highly-trained runner, you may be able to get away with adding more miles more quickly, especially if you have already run a higher level of mileage in the past. If you're a new runner, an older runner, or are prone to injury, run the same mileage for 3 to 4 weeks before increasing it. Give your legs a chance to adapt and habituate to each level of running before increasing the level.
Back off the volume by about one-third for one recovery week before increasing the training load. For example, if you have been running 30 miles per week for three weeks, back off to 20 miles for one week before increasing above 30 miles. Think of this strategy as taking one step back at the end of each training cycle so you can take two steps forward during the next one. Over time, your weekly mileage progression will look like this:
Weeks 1 to 4: 25-25-25-17 miles
Weeks 5 to 8: 30-30-30-20 miles
Weeks 9 to 12: 35-35-35-23 miles
Weeks 13 to 16: 40-40-40-26 miles
The weekly mileage systematically increases over time, which is the key for adaptation and preventing injuries.
More: 7 Secrets to Prevent Running Injuries
Given the link between aerobic development and running success, the base-building phase should be the longest phase of your training program. It takes longer to develop endurance than to develop speed. Depending on your starting point and your running goals, you may want to devote eight weeks (experienced) to 16 weeks (beginner) to base building. If you're training for a marathon and you've never run before, you may want to spend an entire year on base building.