I am glad that you recoginize the importance of flexibility. It is important as people get older that they spend time working on their flexibility because that will decrease their chances of injury. I think that you should focus on full body flexibility and not just touching your toes. The American College of Sports Medicine says "exercise programs for elderly persons, as well as other populations, should emphasize proper stretching for all the major joints, especially for areas affected by a reduction in range of motion." When stretching remember to:
- procede streching with a warm-up to elevate muscle temperature
- Do a static stretching routine that exercises the major muscle tendon units that focuses on muscle groups that have reduced range of motion
- Perform a minimum of 2-3 days per week, ideally 5-7 days per week
- Stretch to the end of the range of motion at a point of tightness, without inducing discomfort
- hold each stretch for 15-30 seconds
- two to four repititions for each stretch.
As for which stretch to do, the bent knee stretch may be the best to start off with because it may cause you the least amount of discomfort.
I hope all goes well for you.