I tried boxing when I was in my 30's. I was very fit and strong, but incredibly slow of reflexes. A buddy, a former US Army Golden Gloves fighter was re-entering boxing, primarily as a way to get into the new sport of bare knuckle fighting. He needed a sparring partner. I got the snot beat out of me for about six months before giving up. It was about the hardest sport I ever attempted..
You are right to get into shape at 50. At 60, its a whole new ballgame, so do it now! I re-entered cycling a few years ago, and that has become my passion and means to fitness. I've been in the restaurant business for over 40 years. No one ever says "hey, look how fit that retired restaurant guy is". I vow not to be that guy!
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Don't know what exactly it is about boxing practice which drains you so much, also sweating so bad you literally feel glued to the mat. I haven't done another proper combat sport but I've played basketball and football (soccer...) before, a lot of weightlifting. I think it's a combination of both physical and mental/emotional challenge. It looks simple from the outside and literally beats you up when you're in it!
Only thing coming close was an amazing thing I did when I was in the UK called BritMilFit. It was run by former military men and women, done in absolutely any weather. Was in essence a HIIT, but these people managed to strike an amazing balance between challenge, being interesting and being funny. They had a bit of a Full Metal Jacket style to them, but never offensive. I think only Brits can pull this balance off. Nothing's came close to being a fun yet very challenging exercise for me. I think trainers were hoping for bad weather, I vividly remember once there was heavy rain, they took us to the lowest elevation point of the park, had us form a line, legs spread, the last person on the line had to crawl under everyone else, once they were out the next one would go. Two teams, losing team did jumping jacks, winning team did pushups, then go again. Guaranteed crawling through the elbow-deep mud, was so much fun and liberating! Only bad thing was just how messed clothes would get at the end of it.