If you feel you've done some damage to your body, then get yourself checked out. And make sure your doctor clears you for (intense) exercise, both resistance and cardio.
Walking is good, but alone is not enough, especially at your age.
Visit sites like cbass.com and baye.com (and do searches for topics of interest). Admittedly, these are HIT sites, but they're a good place to start, even for people who believe in volume training. The idea is not to bite off more than you can chew, especially at the outset, which would discourage you. Read Body By Science, by Doug McGuff. I think he understates the amount of exercise needed, but it has excellent information on which to build a suitable routine. Visit exrx.net for brief videos and explanations on proper exercise form.
As I wrote earlier, don't do too many exercises; focus on a relative few key compounds. Until you work up to HIIT cardio (if that's something that even appeals to you), walking briskly will suffice as cardio for a time. Use the stairs. Go up them quickly or two at a time. Look for ways to move during the day. Get up out of your chair and walk around the house or office (and up and down stairs). Etc., etc.
