Note: there will be a little weight-loss talk in here, so please feel free to skip if that's triggery for you. And this is all entirely my own experience; I am not a doctor, and cannot claim that this would work the same way for you. Bodies are different! But perhaps it will be helpful.
For about twelve years, I had the same pattern re: exercise. I would decide to start exercising. I would try something -- running or lifting weights or elliptical. The first day or two would be okay -- there was no 'runner's high' or anything like that, but I didn't feel too bad. I pushed through it. But by the third day, I would be exhausted and feel like I was going to be ill after each workout. I would keep doing this for a few more days, but usually after 1-2 weeks of it, I would give up, because it felt so bad.
About eight years ago, two things changed, which completely transformed my body' experience of exercise. The first was weight-loss, the second was a different approach to exercise. Since they happened at roughly the same time, I can't quite separate them enough to know how great a proportion each contributed, but I suspect both together are most effective.
I had gotten to weigh about 50 pounds more than my doctor-recommended weight, a combination of a slow hypothyroid-induced creep over a decade or so (the early part of that undiagnosed), and then an added jump in weight at the end, mostly due to heartbreak and depression. (This was the year Kevin and I broke up, and it was awful.)
At 5'0", 50 extra pounds (almost a third of my total weight) is really physically noticeable. Aside from the aesthetic / clothes-fitting issues, it was getting hard to haul my body around. Climbing up a flight of stairs tired me out. Walking more than a few blocks tired me out. The extra weight was clearly impairing my quality of life on a daily basis.
I wish I could say that's what prompted a change, but it was actually coming out of heartbreak, finally wanting to date again, and deciding that even though I wanted a partner who would love me no matter what I looked like, cultural/biological programming meant that if I looked more conventionally attractive, that would mean a lot more potential partners would even notice me to begin with. Which meant losing weight.
I don't want to sidetrack this into a discussion of weight-loss methods, so I'll just say very briefly that *for me*, calorie-reduction works and carb-reduction in tandem with that works even better. Your body may differ. I spent three months carefully counting and restricting, aiming for losing about two pounds a week, and lost about twenty pounds, which have stayed off (barring pregnancy-related gain and loss) for eight years now.
So that helped; having less weight to carry around made it noticeably easier to move, and meant that when I started exercising, I didn't feel as immediately horrid. But I think it was part two that was critical -- I changed how I approached exercise.
I started with walking. Just walking. Trying to walk at least thirty minutes three times a week. Once my body was used to that, I upped the exercise level a bit. Going a bit faster on the treadmill, or upping the incline. Adding a little weight-lifting, and increasing weights. But gradually, gradually. Really tiny, incremental changes, but trying to keep it consistent. Never pushing it to the point of feeling really crappy afterwards. Charting what I was doing REALLY helped, actually writing it down.
What I found surprising, even bizarre, is that my body's relationship to exercise changed radically. After about�two months of this? I found that my body had started really liking the exercise. I felt good after doing it. Not quite a runner's high -- I wasn't pushing the aerobic stuff enough to have that kick in, I think. But my body just felt pleasantly good. (I had more energy for working too; taking half an hour out of the day to exercise more than pays for itself in increased productivity, in how much I get done in that day as a result. It's weird.)
And after three months, I found that my body now physically craved the mild exercise -- that if I didn't get some in (busy computer-work day or whatever), I actually had trouble falling asleep at night. Sometimes to the point that I had to get out of bed and go down to the treadmill and do thirty minutes at a fast walk to tire myself out enough to fall asleep.
I'm not physically there right now -- I get hit hard by winter colds, and then I don't exercise for weeks, and my body gets unaccustomed to the exercise again. But yesterday I started again, with twenty minutes of medium-intensity swimming, and oh -- my body felt GOOD afterwards. I can still feel the effects of it today, in fact; I slept well, and woke up with my body feeling better than it has in a while, and I am looking forward to doing some kind of workout later today. It will actually be a treat in the middle of a long computer-y day.
That's what I find so strange, still -- the me of ten years ago wouldn't have believed that her body's relationship to exercise would change so much. When I heard people talk about how exercise made them feel great, it seemed like they must be lying, or somehow fooling themselves, or else I just had a broken, faulty body compared to theirs, which made me angry and despairing.
But now I kind of love exercise. My body loves it. If I could go back to the me of high school and show her how this works for us, I would, because being more fit and capable has made me happier and made my life just generally better.