To become more flexible, find a good older adult exercise program. Try a chair exercise class, especially if you haven't had an exercise routine for a while. A good instructor will take you through exercises that are optimal for seniors. They focus on "oiling" your joints, strengthening you and giving you more endurance. You may be sore for a day or two after, but you will eventually start to feel more flexible and energetic. Oh, and a good class will help to dissolve stress and will leave you feeling healthier emotionally.
After years of inertia, I started with general chair exercise, then added chair Pilates. After four months I am now learning mat Pilates and am working with a personal trainer on machine and free weights. In the process, I've lost 14 pounds and have regained the ability to bend, stretch and kneel with more ease. I can walk longer distances and hope to take up hiking next year.
Although the quality and class offerings of local YMCAs seems to vary, the Y is a good place to start to look for senior programs.