How to best utilize training plan when running on "rest days"?

This may sound a little odd initially.

I just started my training for my first marathon. My wife also runs. She runs slower than me but I will take a run with her over anything else. I am getting more and more of easy runs on my schedule and I believe it is because I am doing more running with the wife.

How can I properly feed the duck so I can get "proper" run/training recommendations from the training plan? My current feeling is that my long runs should be 8-10 miles but, this Sunday, I have a 39 min base run.

I dont know if this even makes sense to you guys