I am trying to be a "runner"...working on it!! I started in January and pretty much have been doing 5 days a week. I would cheerfully add another day or two but I know it will benefit my body more to
1. Take a rest day...body needs to recovery to get better and stronger. Avoiding a rest day only increases my chance of injury, burnout, overtraining and body cannot rebuild.
2. I do not at a 6th day to avoid overtraining. Cross training improves my running performace, helps with functional fitness, keeps me from hitting a plateau, strengthens and develops muscles and parts that running doesn't put enough stress on.
I say stick to 5 days and do something else the other day or two. One day do step or kickbox and on your rest day do Yoga.
JMO