Stamina Not so Great

The year before last summer the eldest son and I attended 2 days of "tactical shotgun training".  Even with frequent breaks, the summer heat and activities wiped me out.  I was completely done in.  Now a coworker is suggesting that I attend a 3 day training session learning a certain type of gun fighting based on hand to hand martial arts.  The truth is, I am very interested, but 3 consecutive days is just too much for this old man.  I want some more training, but I should stick to 1 or 2 days at a time.