When I was a little child, I fell in love with playing basketball. When I had some leisure time, I would find a court to play. I ______ been doing this all along. Finally I became a professor. My students often kindly invite me to play basketball with them. Regretfully, I found out that I am too old to be involved in this kind of sports. Twice, I almost got hurt. The students were embarrassed. I switched to tennis playing after I became forty years old and have been doing so ever since. I have never been hurt by playing tennis.
Which word should I use to fill in the blank, have or had?