Don't think I've done more grueling workouts than boxing. I'll share a story of the most fucked I've ever been when training (don't worry this isn't some weird sex story!).
I had been going to the club for a few months and only sparred 1 round I think at that point. My fitness wasn't great still but this day I decided to increase my warm up run. Basically most members would do a couple of laps of the block, roughly 0.75 miles each, before training. I had been doing 1 lap so upped it to 2. Since I hadn't run more than about 2 miles ever before this was pretty tiring for me, but I felt good. Inside I got on with 4 or 5 rounds of skipping. I was getting pretty good at it so liked to do plenty. Then I hit the bags, 4 rounds I think. I really went for it and was a sweaty mess by the end. But what the hell I was almost done. Just groundwork and maybe some shadow boxing.
Groundwork was basically a set list of calisthenics done back to back, the aim to complete each list within a 3 minute round, each round involving more reps of each exercise. I hadn't ever managed to complete all 3 rounds before but I felt I was getting fitter and went for it. After, I thought one quick round of shadow boxing and I'll head home for a lager or 5 since it was a Friday. I was completely soaked in sweat. During my round of shadow boxing one of the coaches comes up to me and says "You're not done already are ya, I was gonna get you on the pads". I'd been waiting weeks to start getting some attention from the coaches so I couldn't say no.
My arms were like dead weights but I did my best trying to show how hard I could hit (watch out for my jab guys it sure does sting!). Anyways a couple of rounds in the head coach comes up, "I want him sparring today. You up for it?". Oh fuck! I'd had 1 round of sparring in my first week or so, was desperate for more, and now when I'm close to collapsing he wants me in the ring. I hesitated a second before nodding yes. Too tired to speak, I get in the ring dreading who I'll be up against. It was a young traveler also called Jon. Around 17 I think, 10 years younger than me at the time. But he was pretty much the same height and build, and man was he fit.
I barely threw a punch, I could hardly stand let alone box and move. After 1 round the coach tells me "You're not here to be a punching bag!". He beat me up good. Coach called a halt half way through the next round. I was wearing a white top which was now almost entirely pink. My nose was a mess.
I climb out the ring and just as I'm ready to walk through the gym and out the door to head home, I notice the entire club are doing their groundwork as a group. And this time one of the coaches had the sadistic idea of using playing cards to determine the number of reps. Ten of clubs, ten burpies etc. I couldn't exactly walk past everyone and out the door, so I had to join in. Getting a few looks from the younger members on account of my face and shirt being covered in blood, I embarrassingly struggled to keep up. It seemed to go on forever. I'm not sure how much I missed at the start but the bastard finished the entire pack of cards. Thinking back I'm not sure how I managed to drive home. I seriously thought I was going to collapse.
Needless to say I went back for more the next week. My fitness improved and I got a lot more sparring in. Eventually, I think about 6 months later, I sparred Jon again. I can confidently say this time we were fairly evenly matched (and I bloodied his nose this time, with that stingin' jab!). Yeh I know he was still just a kid but I gotta boost my self esteem somehow!
Anyways enjoy your boxing!