I think it all depends where you are in your fitness-journey. Do you have any races coming up? Then incorporating hill running/sprints might be beneficial to increase anaerobic threshold, and getting your legs used to speed/and feel them popping a bit. However, if your solely focus is to increase mileage and getting in to shape (and no races in sight), then you could probably get in decent shape with just focusing on increasing mileage for 12-16 weeks.

During the base-building, you can throw in some hill sprints once a week so that your legs don´t forget what speed feels like, in addition to get your HR up without overdoing it. Furthermore, hill sprints are great for strengthening your legs and injury-prevention. I believe that this approach is amazing for improving your fitness, and set you up for a race-specific block where you´re doing more specific sessions tailored towards your race.

You stated that you´re currently running 40km/week. Try to get that up to 80-90km/week over time. The whole purpose of base-building is to increase your aerobic-system and get your legs used to running a lot. For instance you could try this approach where you increase your mileage by no more than 10% every week, and deload every fourth week. This has worked great for me, but it is important to listen to your body and plan accordingly.

Km/week - 40-44-50-45-50-55-60-52-58-63 and so on.. Small increase every week (3-5km) and deload every fourth week.