Well, you've just added an extra load into your training week. My legs are often sluggish when I have a heavy week or two. It may just be that. You should get used to a new level of load, but it might be an idea to cut back before your actual races.
As to helping with the training. It should help overall fitness, and is a cardio-vascular workout too. It will utilise some of the same muscles, and some new ones. However, you can't get run-fit by just biking, so make sure you keep up the running training.