TAMPA - It's been on Mary Norris Vincent's mind for quite some time. ``What is the purpose of my life?'' she wonders. ``I'm like a lot of people. We try this, we try that, and we're still not happy.'' The 65-year-old Sun City Center grandmother figures it's never too late to seek the answer to that timeless question. So last weekend, she joined tens of thousands of Americans beginning a 40-day program of group discussion, worship, study and service to find her purpose and put it to work. Nationwide, members of some 5,000 churches - including nearly 40 in this...