I would suggest that you check into less developed regions of the country such as mountainous regions. These frequently connect towns either by the interstate--the quick and possibly longer route or by forested over the mountain and through the woods route. This would also be a beautiful trip.
When I was little, we used to travel from Salt Lake City, Utah to Tacoma, Washington (965 miles) every year for Thanksgiving. If you're familiar with the song, "Over the River and Through the Woods to Grandmother's House We Go," then you will understand that that's literally what we did. The trip took 14-16 hours. We'd leave after my father got home from work, travel through the night, and arrive after dark at my Grandparents. Along the way, we traveled through numerous mountains and forests. Tacoma itself is surrounded by dense, heavy forests.
We also used to go from my home in Salt Lake City, Utah to my uncle's ranch in Sims, Montana, a distance of about 550 miles. It took us all day to get there. We traveled up through Idaho, and though not as heavily forested as Washington, it was truly forest.
Here in Utah, we don't have large forests when compared with other states, but if you travel from Salt Lake City (northern Utah) to St. George (southern Utah) you travel through some beautiful forests! From the northernmost edge of Utah to the southernmost is about an 8-hour drive. Northeastern Utah has tall, stately trees covering hundreds of miles and there are many mountain ranges filled with trees. Salt Lake City itself is surrounded by mountains and in those mountains there are beautiful forests of fir, spruce, aspen, maple, and pine trees.
I am looking at the idea of "journey through the forest." I do not think there are any two towns in the United States these days that meet this criteria.
I think you are saying that you are looking for pairs of towns where it would take all day or all night to get from one to the other and where they journey between the two would be in forest.
With the types of highways we have, a whole day's journey (say 8 hours of driving) is going to be around 500 miles. There is nowhere in the United States where you have 500 miles of unbroken forest these days.
There are, of course, many examples where you have to pass through forests to get to a town. For example, if you take the correct route, the drive from Seattle, WA to Wenatchee, WA is mostly through forest. But it is only 140 miles.
I will give you some information about states which you can cross through forest lands, but you mightwant to specify and re-state your question as to a) which region of the USA, b) are you saying towns, cities, or states? c) what is the purpose of the question (so we can add more detail to your answer).
a) Some states you can cross within one day are
North Carolina (Fayetteville) to South Carolina (South of the Border) to Georgia (Columbus) and Alabama (Phenix City/Mongtomery)
Florida (Orlando) to Georgia (Cussetta, Tifton, Columbus, Albany, Newnan, Johnesboro, Union City, and Atlanta
New York (Manhattan) to NJersey to Connecticut (I've done althe way to Waterbury)
----If this is the info you were requesting, I could edit and add to my answer. Meanwhile please expand with more info.
Rhode Island is one of the best states to find forests that are acccessible from differnt towns due to its size and location. You could walk from East Greenwhich to Coventry to North Kingstown in a day. Rhode Island is a very littlre known state that has much to offer.