The OLP is a fairly easy 27 mile loop northeast of Williamsport, PA.  As its name indicates, most of this trail is routed along old logging grades which allows for fast walking.  There is one or two intensely steep climbs along the trail though.  Plan to see plenty of wonderful mountain streams and possibly a good amount of wildlife too.  Also be prepared for at least one or two moderately difficult stream crossings.  I've hiked this trail several times on one and two day adventures and have always had a good time.  In general, if you can walk 3 miles per hour for 9 hours, then you can hike the OLP in a single day.   

Check out the DCNR website about the OLP:  http://www.dcnr.state.pa.us/forestry/hiking/logger.aspx