Amplify .7Vpp square wave to 5Vpp square wave buffered to go over a 15-50 foot cable
The circuit you're using is able to drive the cable very quickly to the low level, but only pulls up slowly to the high level because R1 limits the current available to charge up the capacitance of the cable. The maximum current you're going to be sourcing into your cable is about 3.3 mA.

One solution is, instead of buffering with the Q1/R1 combination, use a buffer/driver IC such as SN74LVC1G126. This can supply up to 32 mA current either sourcing or sinking. You'll have to work out from your cable length and capacitance per foot whether that will give you adequate edge speeds for your application.