We currently live in a moderately rural/farming area and have 2 acres and some small livestock. I'd love to find a similar area but I was getting a bit confused on the real estate sites.
I am not in Allentown but near, in the ABE area.
Frankly you will not find a web site that will help you- it's best to visit the area. You no longer can tell where Allentown ends and Bethlehem starts but there are areas (such as Trexlertwon, Alburtis, Kempton,Whitehall,Northampton etc)- all outside of Allentown on the western/northern edge that are "close". MOST people drive so if you live even 30 mins away that is really considered good. Most I know drive 45 mins and still work within ABE, there are tons that travel to NYC and Philly (2-1/2 hours each way).
There is some farm land left depending on what you are looking for.
There are great sections within the city as well- Cedar Crest and Tilgham but you won't find farm land left there.
A lot depends on how far you want to drive -it can even take you time if you live on the western edge and need to go towards downtown or ABE airport area.
WIthout more details (section you need to work in) it is going to be hard for you to get answers. Both 78 (for some reason it goes in streaks where they have a major shut down for hours daily!) & 22 can be nightmares at time! IF you move to the area- learn the back roads- DO NOT DEPEND ON 78 & 22!
I grew up in this area and would also be happy to answer questions. Let us know where the job is and we can steer you in the right direction. You may also want to check out the north eastern section of the valley (forks township) as there is still some farming land up there, but it is being taken over by subdivisions.