Many come into running (or triathlon) without taking the time to build a base and end up injured when they try to do too much. Building a good base allows you to develop the muscles, ligaments and tendons to support you properly while you run.
The other (and as important) reason for a base is it serves as the physiological foundation upon which the rest of your training can be built. The bigger the base, the higher you can build your fitness. The higher you can build your fitness, the faster you can go!
Take the traditional route
You could do primarily aerobic running, keeping your heart rate between 75-83% of your threshold (or slower than marathon pace). The general rule of thumb then is to build slowly. Try increasing volume by about 10% a week (allowing a rest week every 3-4 weeks) once you feel able to do more.
This method means you do about 80% of your training within the aerobic zones. The rest is completed at a high intensity. Until you’re ready for the higher-intensity work you could incorporate some strides (where you reach race pace over a 50-100m distance) during the last 20% of your training. This might look like 10mins warm-up, 20mins with 3-4 strides (3-4mins aerobic between them), 10mins cool-down.
This is good for us triathletes as we already reap run benefits for the work we do on the bike. It applies to other modes of exercise too. Hillwalking strengthens the legs/ankles/feet, while the elliptical/cross-trainer at the gym is a good substitute for running. It will allow you to increase volume or do some higher-intensity work with a lower risk of injury.
Does hiking help strengthen my legs for running and cycling?
Once you’ve built a big enough base to tolerate some interval training, you can get more specific for your sprint races. This can tie in with the polarised training if you choose to go down that route. Start small and build up your interval sessions but keep your aerobic runs in there as the majority of your training week.
So, as it is sometimes said in the triathlon and endurance sports world: run often, sometimes fast but mostly slow. Take the build slowly, it’s a work in progress.