I just took a quick read through of The Next Generation Companion opening chapters leading up to airing of Encounter at Farpoint, and I can't find any references to TNG being set 78 years after TOS.
There's mention of 100 years, 200 years, the 24th and the 25th centuries, but no 78.
The promo material release says, 'The 24th Century Adventure is about to begin'.
That's the advertising I remember seeing in my local newspaper and TV Guide. I think even the commercials only mention the 24th Century.
If I still had my FASA TNG Officer's Manual, I think that might be the first mention of it being 78 years after TOS.