After last summer's kayak trip I realized how important flexibility is going to be to my staying active as I grow older. More than strength it is what will allow me to adapt to new sports. So this fall I started doing hatha yoga twice a week. Flexibility requires dedicated consistency. If you don't consistently stretch the tissue it won't retain the ability to stretch.
Fitness centers offer plenty of stretching classes.
There are good books about stretching. Take a yoga class or a streching class and then strech regularly. For kayaking emphasize forward bends that strech your hamstrings. Watch your technique to avoid cheating by rounding the back. To be effective the strech must come from your short hamstrings, not your already hyper mobile low back.