Okay. Lets see what I can do here.
First off, I get NextLevel's point that if the OP is asking about exercises that will help his game get better then probably time at the table is the most important thing for the person.
Second, I get that, from the standpoint of just getting better, an hour of serving will help most people improve more than an hour of crunches and strange foot patterns of putting your feet into a box and out of that box.
However, the work I do, to a lot of people is about getting in better shape and being healthier. It is not really how I think of Yoga but it is an undeniable part of how many people in the west and many people who take my classes think of what they get when they take a yoga class. I also teach many of my classes in gyms where there are other classes that are, without question about nothing else besides fitness.
So, I know a lot of people who will simply go to a weight class, then an abs class, then a ride class (stationary bicycle), and they do it simply because they want to and simply because they want to get in better shape. Now, the people I know who really know how to work out, they go to the gym and they just work out. They don't really go to group fitness classes. They just know how to work out and they do it. I like other kinds of working out aside from table tennis and I don't really consider yoga working out. I consider it an internal process even though it has a physical element to it and it can be made to be physically challenging.
So, those people I know, who want to get in better shape who take those group classes, I don't tell them, go learn how to work out on your own, you will get more than you get from those silly classes you take. They like the classes. It is fine for them to take the classes. For them it may even be better than trying to learn to work out alone.
Okay, so I have gone off on a tangent but I think the tangent is worthwhile and useful. I plan to go on another few before I end this.
The point I am making is: IT IS ALWAYS good to workout and do fitness stuff. Or, at least for more than 90 percent of people who have normal health who are between the ages of 20-70. Most adults can benefit from more working out and more physical exercise. And physical exercise actually works better for your health when it is sufficiently varied. Your body gets smart and if you don't keep changing what you do, if you don't vary how you workout and use your body, it figures things out and stops getting stronger and faster. This is also part of why cross training is valuable. So even though the points about specificity are accurate and a pro basketball player who is in amazing shape can still get toasted by some evil multiball drills that I would handle just fine, cross straining still has value.
Now, if a normal person could spend 6-7 hours in a day playing table tennis, they might not need other exercise on a day like that. If I could play that much on a regular basis these days I would be much happier. And yes, that would probably also get my level higher, faster than working out would.
But I am not going to try and read someone else's mind or life situation. I am not going to try and figure out the exact circumstances of the OP and I will go with the idea that working out is good for you and intelligent, efficient workout routines may be the fastest way for most improve their physical fitness level and their cardio endurance level.
Yes: someone who plays TT all the time, who is 800-1700 USATT, who does not have a physical limitation like RA, if they were given a fitness routine that was 1 hour long that they could do on their own or be guided through, 1, 2 or 3 days a week, doing that routine could help their fitness and cardio endurance levels improve much faster than their FITNESS would improve with 6 hours a day of playing table tennis with people OF THEIR OWN LEVEL.
From a fitness standpoint, if I play with a coach like Paul David for an hour I get a pretty decent workout. If I play 3-4 hours of training with another player my level, I still will not get the amount of physical workout I get with Paul David in 1 hour; in truth, if I play 7-8 hours with someone my level it still may not be as much as I get in an hour with Paul David, especially if we go all out. If I workout in the gym for 1 hour, I can get as much or more of a workout than I can get in an hour of all out training with Paul David. Part of why is that I know how to workout.
So, here is a scenario. It is Tuesday night. I finish teaching at 8pm. I could walk home and do nothing. On Tuesday my last class is a 20 min walk from home. I could get on a train, go to Manhattan and play at a club for an hour or two. Max table time would be 90 min with a guarantee of waiting to get on a table. If I do that I get to the club by 8:40pm. I leave the club 10:40pm. I get home 11:30pm. And I don't get much play time and I get home sort of late. Or I could stay at the club I finished teaching at, work our for 90 min, and get home before 10pm. The workout would be non-stopped. I would go from exercise to exercise. Do some running on a treadmill for maybe 20 min. I have a way of working on a treadmill where I spike the speed for 30 and 60 second sprints and then drop the speed back to a 10 min mile. This method is really efficient for increasing cardio endurance. Then I do some shadow footwork drills (lefty and righty), mix them with ladder drills, abs, pushups, and go from exercise to exercise, so that, when I hit a wall and am at max, I switch exercises so I can keep working at full capacity but change what body parts I am working. After about 40 min of that, I can hit weights for 20 min and then finish with 10 min of stretching.
From the standpoint of physical fitness, I did the equivalent of 6-8 hours of what I would get if I had gone to that club to play for 2 hours. From the standpoint of amount of fitness training, it would take 4 Tuesdays of going to the club to get the same amount of fitness workout as 90 min at the gym. And that working out, would crossover into my ability to play table tennis to some extent even if it did not do as much for my table tennis skills as going to the club would.
So, from the standpoint of efficiency, if my goal is nothing but getting better at table tennis as fast as I can, then, for a player under 2000, NextLevel has a point. But from the standpoint of my life, I would take the workout 70 percent of the time. A few years ago, I would not. I would have taken the table time. So I get that. But today, thinking of the whole scope of my life and the value of getting home to be with my family earlier, I would take the more efficient workout that is not directly about TT but can have some value to my TT skills anyway, over the one that is directly and immediately about TT.
What is the point I am really making here:
Often it is not the best thing to do to discourage someone from doing something that may be healthy for them like working out.
If the OP's question was, "what is the fastest way I can improve in TT," I think NextLevel may have the right answer.
But since I choose to hear the OP's question as: "I want to workout to supplement my TT, I want to do stuff that can help improve my overall fitness level but I want it to be stuff that is useful cross training for TT as well," FROM THAT STANDPOINT I find it wise to encourage that impulse rather than discouraging it.