1nF and 800Khz is bad idea. XC would be 198Ohms at 800KHz, but the edges are basically higher frequency armonics, so it'll make a nice low-pass filter, stripping the edges and causing the signal to look like a sine.
Proobably the capacitance from the wire itself will filter things out, as the signal from a ttl is weak.
Your circuit is
I'd try loading the end of the wire with a resistor to avoid signal reflection, trying different values.
Having scope would be great for checking the signal integrity.
It's hard to guess, such speed and lengths are differential signaling territory.
That driving circuit, it's already a miracle if it worked even at low distances.
800KHz and 100K pullup? Thats a very weak signal!
Get a 74LS04 (74LS14 if you want Schmitt trigger), LS series have lower voltage threshold than HCs (~2.7 vs ~3.3V ), higher current capability, and you could parallel them for higher current (sharper signal).
Also 74xx ics are extremely inexpesive.
Remember to put some 100nF decoupling capacitors at the VCC pins to help with the transition current spikes.