try sleeping with your legs raised, it helps the excess water to drain back up them. cut down on your salt intake as it makes you store excess water. being seditary does not help, but it only takes a few minutes an hour standing/moving around to eleviate swollen ankles. I can't stand at all, but move the position of my legs every so often and unless i am on my mobility scooter all day (6hrs+) my legs rarely swell.
another thought is your socks may be too tight they definitely will be when your ankles/legs are swollen. try to wear non elasticated ones or go without in the house. elasticated leg bottoms on things like joggers could also be the cause.
going to the gym sounds like a great idea, but even just a walk around the garden will help.