Been a little while since law school and this isn't my field nor State, but here's a rough idea as to what you're looking at. This is general easement law, and could have been modified from one State to the next.

An easement is a very limited property right. Easements can be express, implied, by prescription, or, in rare cases, statutory.

An express easement is one where someone actually conveys an easement interest to another, by deed or otherwise. If you had an express easement from this purchase it is because the owner had literally been given an easement from this guy, in writing, and then he grants it to you.

An implied easement comes in two varieties: easement by existing use and easement by necessity.

Easements by existing use occur if there was common ownership between the two tracts of land (i.e. the guy sold off the property to the current owner), and the property owner would consistently use the other's property in a way that was very visible and necessary.

Easements by necessity only occur if there was common ownership between the two tracts of land at one point (i.e. the guy sold off that property to the current owner) and at the time that the property was severed (i.e. he sold part of it, the 20 acres you want to buy), then there was a strict necessity for the use of the other dude's property to access the property that you desired.

An easement by prescription occurs when one landowner continuously trespasses on the other guy's land and the other dude never decides to exercise his right to prevent that person from doing that. Easements by prescription may be passed from one landowner to the next.

Statutory easements are easements simply created by statute, who knows, maybe there's some New Hampshire law that creates any easement like this Arkansas guy says about not being denied access?

Anyways, in short, what seems to be most important is the actions/activities of the prior owners. In any case, yeah, you need an attorney, the above is general shit that law students learn and it can vary from jurisdiction to jurisdiction.