As some general advice from someone who runs half-marathons, I can offer this information…

I actually think it’s a good thing that you’re running lower distances than the 21K you’ll be racing at. The newest school of thought is that, unless you’re a high-performance runner in very competitive levels of racing, you should seldom actually be training at your racing distance. 21 kilometers is a lot of mileage, regardless of experience, and frequent running at that length can be damaging to your body’s integrity. At most, I would work up to training at the full length once every one or two weeks.

I would also replace some of the other running days with resistance training. A lot of runners disregard weight lifting as being a necessary component to their training, because the thought is that running means you don’t want to be bulky. You’re not going to “bulk up” accidentally, though. What you’re going to do is build the necessary strength that your muscles need to be able to endure repetitive contraction and impact over a couple hours. You also need to train your muscles’ explosiveness. While long-distance running isn’t as much of an explosive, power sport as sprinting is, each time your foot is leaving the ground is a small-power contraction (power as opposed to the contractions in slow strength movements), and so you should be supplementing your training with exercises that will train that system. (Bounds, box jumps, maybe even Olympic lifting.)

As for the rest of the days you run, have a good mix of shorter distance, slow-paced runs (10-15K) and much shorter, higher-paced intervals runs (6-10K with sprinting intervals).