I’m sure exactly what Nordic Walking is but I’m going to guess that it means you are using poles and perhaps taking a bit longer strides than when hiking. In XC ski training this is often used for training and is called Ski striding.
If you are training for a running event then you should run as much as you can while staying in the appropriate HR zone. Nordic walking up the steeper grades where running is too hard is a fine thing to do but if you can run on the flats I suggest doing that.
I hope this helps