Find the time to not work
I know, I know. I am supposed to be giving advice on the working life, but part of that is also the not working part. You can arrange your schdule to get as much work done as you can, but you must also find the time to exercise, and stick to it. It is so easy to let it slip, to say, oh I have a deadline, and I can’t make it to the gym, but if you do, you will find yourself going less and less often.
I don’t go to the gym. I dance twice a week, because that is what I enjoy, and I know I can stick to it. However, it is very hard, with deadlines looming, sometimes, to get to my dance class, but I do, either by working when I return late in the evening, or getting up extra early the next morning.
Nothing should keep you from keeping in shape, no matter what.