Hardest I have ever worked out without a doubt was training Thai Boxing in a Bangkok back street gym.
I thought I was fit before I went over, and then I got introduced to the 4 hour morning session…..fuck that shit, 15 minuted warm up, then 2 hrs conditioning, including sprints, squats, push up and various other boxing related torture, followed by working the heavy bag followed by sparring. Then have the rest of the day off and then there was supposed to be a 2 hr arvo session the same day, the locals were training both sessions every day.
We alternated between arvo and day sessions depending on how we felt, never managed to do both in the one day.
Never again have I been so fit or lean in my life than after a month of doing that.