I've done that in Ohio, except I brought a light fleece blanket. It worked OK, but nowadays I bring a 45-degree down quilt - lighter, and a little extra margin in case the forecast is wrong.

Speaking of which: where is that 70 degree prediction centered? Usually, that's a prediction for a nearby town. Depending on such things as elevation you'll be at relative to that town, exposure to wind, any rain predicted, or microclimates that you might encounter if you camp by a creek, that 70-degree low may actually end up being 60, or even 55. (Bill S: please chime in here, since you have direct experience in those locales.)

I'd probably give it a try at a car campground before going out in the boonies - if it doesn't work, you hop in the car and drive home (or better yet, pull out the sleeping bag you stuck in the car, just in case.)

Let us know how it works.