I can only recall flying one long haul flight back when smoking was still allowed.. SYD-DPS for a Skydiving comp of all things...
IIRC the "Smoking zone" was toward the rear of the aircraft... which of course meant that it was possible to be in the last row of non-smoking... right in front of the first row of "smoking"...
I can't recall it being too awful.. but then at the time we were quite used to smokers being nearby on trains/buses etc..
I think these days it would be WAAY more noticeable...