It's so important. I'm working through it and I honestly think that embarking on a health lifestyle, both in diet and exercise, is the #1 most important thing a person needs to do if they want to beat an addiction. You really need to get your brain's natural mood chemicals working again normally as fast as possible, and eating healthy and working out are major stimulators of it.
When I was 20 years old I worked out all the time, I was really strong, I could bench press 240 pounds, I could deadlift 355, I weighed 185 pounds. Today, after four years of opiate abuse, I'm about 140. I've eaten so badly. I never eat breakfast, I buy something crappy for lunch, and then I'm too lazy to make dinner when I get home so I do something pathetic like drink a glass of milk and eat a sleeve of crackers and call that a day.
It makes your complexion really bad, I'm constantly constipated, There have been a few times where I'd taken opiates and then I smoked a cigarette and got so dizzy and light headed I almost lost consciousness, like I had to go down on a knee or I'd fall - It scared me so much at the time, but now I think that was probably it, having these drugs go into your system on a stomach that hasn't had anything put into it for 16 hours.
Getting in shape isn't even hard. If you can get to the gym twice a week and lift weights for an hour, you can build a great physique. You don't need to go crazy with planning a diet, we all have a sense of what is healthy food and what isn't, pick the good ones. I'd say the most important clear rule would be to avoid eating sugar. Of course moderation is ok, but really I think a person would do well to eat as little as possible. Eat lots of fruit and vegetables and lean sources of protein like fish, chicken, whey powder, and then oatmeal is probably the best food there is.
The hardest part is just getting started. Once you do, it becomes easy because it makes you feel better and you'll want to continue. It is a true natural high, exercising. There have been times when I'd had a really good workout and I'd be relaxing in the steam room after and taking a shower and I'd feel such a clarity, great mood, it's almost euphoric.
Don't worry about strict diets and workout routines, just focus on being disciplined in actually getting active and eating well and it'll take care of itself.