Primaries as we know them didn't really exist before 1968. Some states held primaries and caucuses but not all, and the nomination was still decided at the conventions up until then. Even in 1968, the primaries themselves couldn't get enough pledged delegates for the nomination outright before the convention.

That year, Eugene McCarthy declared his candidacy on November 30, 1967, which was about three months before the New Hampshire primary, held on March 12, 1968. McCarthy got 42% of the vote in NH, which prompted Robert Kennedy to jump in the race for subsequent primaries a few days later, on March 16. LBJ then dropped out of the race with a televised address on March 31.

On the Republican side, Nixon had been organizing his campaign by mid-1967, but he did not officially announce his candidacy until February 1, 1968. His main opponent was Michigan Gov. George Romney (Mitt's dad) who announced his candidacy on November 17, 1967.

There were no debates during the primary season on the Republican side, and only one on the Democratic side, in the summer of 1968. Debates didn't become a regular feature of Republican primary season until the 1980 election, while they had been a feature on the Democratic side in non-incumbent years since 1956. And while there had been the famous Kennedy-Nixon debate of 1960, debating didn't become a regular feature of the General Election campaign until 1976.

So, yeah, they were shorter in the past. It wasn't until the 1984 election that candidates began announcing their candidacy a year out. And as late as the 1992 campaign, Bill Clinton didn't announce his candidacy until October 1991. Likewise, George Bush announced his candidacy in October 1987, and Reagan in November 1980. But Mondale, Dukakis, and Dole had all announced their candidacies a year out. The 1988 Democratic nomination was the first campaign where a debate was held in the calendar year before the actual election.

Depending on your criteria, you could probably point to that one as when the modern style was born, but again, even in 1992, the eventual nominee didn't announce his candidacy until the October before the primaries.