Do it. I have an auto immune disorder that kicked me in the teeth after not making up its mind for about 12 years. I just turned 30. Luckily I was in awesome shape when it happened. I haven't been able to work out since last October, and I'm living off 3 years of intensive working out 5-6 times a week. Hoping it gets knocked into remission soon. The only reason I didn't gain weight from inactivity (well, I did eventually) was because they misdiagnosed me and I lost 20 pounds during the summer. I was in optimal shape, so I basically lost 20 pounds of muscle and was about to enter the stage where dying of malnutrition was becoming a concern.
You never know what your body is going to throw at you when you get older. Stuff that has been going on goes awry. Women often have thyroid issues, and losing weight then becomes a problem...
Eating healthy and exercise regularly is pretty much the best way to go about it. And calories. Just manage calories. Reducing intake of things like salt and fat helps calorie management, but ultimately you do those things to prevent diseases related to having too much of them in your diet, not weight control.
You don't have to run 4 miles a day, lift weights and do aerobics 5-6 times a week like I did to stay in shape. Running 3 times a week, watching your calories and just making sure your metabolism stays pretty high keeps your body pretty fit.
And of course, remember... slow and steady. I don't really believe in crash workout programs, although if you're being paid for it like celebrities you can pull it off. Most of us have work and lives to live, so you gotta fit in whatever works for you. Injuries, surgery, diseases... all those things limit what kind of exercise you can do. Some people just aren't meant for running, but you still need cardio. Some people are not meant to develop big muscles because they're just not built for it and have sensitive joints (like me), but you still need strength.
Find a personal trainer, keep with him/her for a month or two. Once you get a routine going, check the interwebs, you'll find something that works. Just remember the first few months are a horror. Especially if it's been years since working out was a routine part of your life.