Boston '82 was very warm. I don't think Salazar was ever the same runner after that race and I think the same was true of Beardsley. Jim Peters was essentially wrecked after his collapse in the heat at Vancouver in the '54 Commonwealth Games. On the other hand, Jack Fultz won Boston in '76 in the hottest Boston ever and was back with a 2:11 in '78.
I'm not sure we all handle heat and humidity the same way. I don't think I ever adapted to running in the heat as I heard would happen as the summer went on. I always felt more drained by and thought I handled it less well in late August than in early July. But maybe that was more mental. I may just have been sick of it.
The four legendary heat trainers that I can think of were Christine Clark, Buddy Edelen, Ron Daws, and Benji Durden. Most of them bundled up and ran in hot weather in sweats. Edelen did it in weather that was cool and was trying to simulate hot weather. Daws got the idea from Edelen and as he described it, both of them piled the sweats on every other day and just ran in normal gear on the other days in locations where the weather was generally cool or even chilly. Daws specifically mentions the need for recovering from the ultra hot runs so neither he nor Edelen were exposed to the sort of daily steam bath that you're getting in the South. Clark was running on a treadmill in a heated room in Alaska so she also was not dealing with oppressive heat round the clock. I don't know what Durden's frequency was.
The Finns always did well at the Boston Marathon back in the days when Finns did well generally. Most of the time they came straight from their winter, it can extend to April easily there, and managed to run well at Boston even when the weather was hot. In 1975 I was in Finland with a British friend and staying with a Finn who'd been second at Boston in 1964. We asked him and some other runners there how they managed to run well in warm weather when they had trained in the cold. Did they do anything special, we asked, to adapt to heat like wear extra sweat suits. The question seemed to puzzle them.
They'd just talk about their mileage and the sorts of workouts they did but didn't seem to grasp the idea of doing anything special to adjust to a hot day in Boston. I got the impression that they were just so cold from training through their winters that running for a couple hours in seventy degree weather was sort of a treat.
At any rate, despite being a Lydiard guy, I'm definitely with malmo on this one. You can find examples of people running well at the marathon without doing any long runs so I think having a few weeks without one is not going to be a problem for you as long as you keep the overall mileage up.