Re: Love training, and I have a heavy build and joint issues.
First off, i would like to cheer you on your perseverance.
Take it from someone who's been at 120kg. Aikido at 75kg is a breeze on the knees and ankles. So the best thing to do is to get your weight down. Proper diet, weight management program, exercise - cardio and weights should get you on the right track.
Meds - not a good idea in the long run. You don't feel the pain doesn't mean there isn't something wrong with your joints.
Aikido in itself should be healing when practiced properly. Try to practice the ki movements more. Misogi, funakogi undo especially.