How to write and do something you don't like

As I start getting myself out there more as a writer I have come across some assignments that could do wonders for my career yet they seem so hard to do.
To those who know me personally they can tell that I am a very picky eater, I do not like to cook and I like junk food the most. So having to write about health and healthy meal options it's quite a stretch. I am surrounded with plenty of friends who are into health, who eat nutritious foods and actually eat vegetables because they like them, they tell me about the benefits of a healthy life style, yet my body is so accustomed to junk that when I start eating healthy my body goes through not so good changes that make me go back to my old ways. I have a hard time eating things with fiber as they send me running. And eating vegetables often give me too much gas. I wonder if this is normal as I must confess that the aroma that comes from me seems to be more of death than health.
I have been encouraged to keep trying, to keep incorporating health into my lifestyle, that once I see the benefits of it, I will not go back. My friend Karina from eatpretty.com told me that maybe I could see healthy foods as a medicine at first that can cure my tiredness, then after I see how energetic I become then I won't think twice.
As I struggle with this dilemma, about how to become more healthy so I can write about health and not think it's a drag, and maybe further along my writing and get more credibility, it seems now at this point in my life starting to live a healthy lifestyle can reap more benefits then before. So I ask you, what do you do when you must do something you don't like to do? Do you have any tips for someone who is having a hard time making the switch to a healthy lifestyle?