I competed in the first UK full Ironman in 1985. I did however have a ten year sound base in cycle racing and about five in distance running. Swimming though was something I’d never bothered with, so I had about six months to get from two lengths to two miles plus.
Basically back then there wasn’t any training plans about, no diet advice, no energy food, no wetsuits, no tri-bars. Just Speedos, steel road bike and a pair of the new on the scene Nikes.
So training was pretty simple as well. Back to back. run – cycle, cycle – swim, swim – run etc. I also ditched pool training and donned a rubber hat smeared myself in vaseline and jumped in the Serpentine.
Basically what I’m saying is quality training and a bit of luck injury wise and you’ll do a good one. The main pressure on me back then was that there weren’t any guys out to just complete it, there were about fifteen entrants and it was a full on race.
Think I got a sub 10 hour. Nearly last out of the lake, posted the third fastest bike time and died on the run because of it.
Edit; My none technical approach to training generally consisted of two 1/2 marathons a month, three day blocks of ten mile runs, 10 mile TTs once a week, 100 miles two Sundays a month. Swimming was squeezed in during any free time as I found I could swim train after competing in runs or rides.