On the exercise front. Don't know what medical or physical limitations you have, but walking may be an option and more effective than you might think. If possible, try to work up to at least 30 minutes and 45 minutes or more would be better. But if you've been really sedentary, gradually build up on distance or time. Don't have to go all out to start. Eventually, you might do both a morning and evening walk as well. Since you're retired, time and schedule shouldn't be a constraint.
On melatonin, less might be more. Instead of a full 5 mg or 3 mg dose, try breaking a 3 mg pill in half and taking 1.5 mg 30 minutes before bedtime with maybe a little warm milk. Other OTC option is Benadryl (Diphenhydramine Hydrochloride). With any sleep aid, try smaller doses first and probably best to not take them every night to avoid dependency. Use sleep aids as little as possible. Not a doctor, but have dealt with insomnia. I'd emphasize the exercise. The body is meant to move to stay healthy and once that is ceded, it's a downhill trip where you'll be boxed in by medical options only.
Good luck.
