Do it. We are about the same age, samish weight and a I am little shorter. I also have a 9' board with similar dimensions to yours.
I bought myself a 5'10 fish, think it's 20" and 2.5", and although I struggled at first to understand the new dynamics of the new board and wave catching, it's been a lot of fun once I got the hang of it.
Ask people for tips on surfing smaller boards too. One of the best I got was to throw your backfoot hand, in my case, my right hand since I am normal footed, forward, this instantly changes your stance to a front foot stance with your weight on the front foot and changes the way the board performs immensely. You can then apply that to your longboard too and watch your acceleration down the line increase. Try it standing up at your desk. Bend your knees, with your hands to the sides of your body in your surf stance. Now throw you backfoot arm, normal foot, right arm, goofy foot, left arm, forward. Feel the shift in weight.

Go for a 6, 6'2 board at your height with some good width and volume. The little extra volume will help while waiting at backline too. I also opted for a quad fin setup which seems to add a little extra bite to the board in the wave.
Do it do it do it!
