Hello
I've just gone back over some of the figures you mentioned.
120 microseconds per analogRead would give over 8,000 samples per second (assuming no other processing).
On the serial side, with a bit rate of 115,200 bps and 10 bits per byte, you could in theory get over 5,700 16 bit integers per second. However, I have never tested Serial to see if it can sustain that rate.
You need 700 samples per second? Have you tried just a simple loop (non-interrupt) that polls the digital pin for a change from low to high, then does the analogRead and then writes the two bytes to serial?
Sorry if I am missing something obvious 
Regards
Ray