Time and patience to be honest...and the vigor of youth.
I was at uni and training the following;
Gym work including weights 3-4 times a week, 1-2 hours each. Mainly doing 5x5 and assistance work.
Cardio out and about; 1-2 times a week - hard run and a light one
Skill training; Muay Thai - 5-6 times a week. 2 hours each
Kick boxing; 2-3 times a week. 90 min to 2 hours each.
The KB was at uni so not exactly difficult.
I think I was doing 20+ hours a week training for 2 to 3 years. A 6 month period was the phasing in the volume.
I think at one stage I was all 7 days, all quite hard as it is hard at that age to tell people to take it easy as this is your 3rd training session of the day.
Worst two days were squats on the wednesday morning, kbing lunchtime, MT hard pads in the evening. KBing on the thursday lunchtime, NT hard pads and sparring in the evening followed by a hard 3 mile run (sub 19 min with hills pace). Fridays were my 'let me just die in peace' day...while doing kbing.
I slept a solid 8 hours and ate quite a bit. The problem was maintaining weight as I was of the 'train to eat' mentality (2 burgers, bacon and cheese at 2300 after coming in from training and then going to bed).
Also; late teens/early 20s and just doing a physics degree so mostly doing that on my on time.
The question is, do you want to feel like a zombie, which I definitely did. A focused angry zombie, but still.
My training log (my second one, rip the great purge of 2007...maybe) will have the lower volume days. 2003-2006 were the above years.