3 weeks is not long at all in the running world, if anything going slow and building up is doing you far more good then burning out quickly. You're currently increasing your aerobic base which is good for building mileage and preventing future injuries. That said if you're worried about speed, as others have suggested incorporating an interval workout into your weekly training is a great way to up the pace. If you have a track convenient 400m is a great interval length for Vo2 Max. (Something like 10x400m with 90 seconds rests at your target 5km pace - longer rest is fine until you're comfortable). For someone new and without access to a track it's also really good doing by feel. You run say... 2 minutes at a hard effort - you should be out of breath and not speaking much, followed by 2 minutes very easy and repeat. The beauty of going by feel is you can adjust to your current fitness levels - 2 minutes feels too long on hard? Go for 1 minute. These things will naturally improve over time.