Not quite the same mental adjustment as diet.
I mean, you have to eat, and unless punishing yourself with a grotesque diet, you will enjoy it. The only problem is staying away from less healthy foods, or portions, that you think you'll enjoy more. Exercise and work are trickier.
You know for a fact that even though you frequently don't want to begin exercise, you almost always feel great about it when you're done (unless you've overdone and injured yourself). So the problem is really just powering through those first ten minutes.
As to the work you have promised yourself you will do when Today Is The Day, well, this too is a question of attitude. How much you have always thought you'd love working on that novel, that popular song, that website from which you will sell the product you understand so well. I'm on your side. I believe that you not only think you can do that stuff, you can do that stuff, and once you get in the swing of it, you will frequently love doing that stuff. But you know, you have to power through the first ten minutes there too, and sometimes it's more than ten minutes.
Once again, I offer you advice that is not original, just good, and proven.
Schedule it. Dress or otherwise accouter for it, go to a place you have assigned yourself to be at for this thing, and stay there doing nothing else for an hour. You must at least start it, and do some small piece of it, and when you stop, you can't leave, but must just sit there until an hour is up. (You could practice meditation, I suppose.) This doesn't sound very appealing, but great multitudes of people have found that it does work. Once you start, you warm up, and it's much more fun than doing nothing. You can't say that the risk of doing nothing for forty or fifty minutes is holding you back, because you frequently do nothing but watch TV or surf the Internet (you wouldn't be reading this else) or play some game you've played a million times or talk to strangers in a bar--and all for hours and hours and hours. Want to break those habits of wasting time? Sit and count the minutes as they go by, with no alternative but going back to something constructive.
The big work project, unlike exercising, which you can just start in on, often has a daunting startup phase that breeds procrastination. You can't write a novel without an outline. You can't build a website for, and market, your housepainting business without researching how to do those things. Okay. The first hour, you make a list of what has to be done. You turn each vague item on the list into something specific enough to do. Get that book out of the library. Call that guy and ask. Read those instructions. Next hour, you go down the list and start doing every little thing. You will get to the good part eventually, and enjoy it all the more when you do.
Oh, one more thing. On the exercise, start small. If you've been sedentary a long time, and it doesn't have to be a very long time to reduce your stamina to zero, you aren't going to do an hour of vigorous exercise. It would kill you. You laugh, but you won't do it, and if you try, it will put you off exercise for another year. And it's not necessary to go a full hour to begin building a very healthy exercise habit. I recommend at least starting with this sledgehammer workout for fourteen minutes. If you never get past fourteen minutes, it will still do you a lot of good. I have no association with the author of that page, I just think he's gotten a lot of things right (the fourteen minute time limit is particularly clever); you may notice that he also has a diet plan, and that I have borrowed ideas from that too. Great minds, etc.