Great decision!
I have a chipped vertebra in my lower back from childhood and simply need a rather upright position.
I've found that the more I ride, the better shape my "lower half" is in and it seems to help "support" the back.
Don't over do it starting out. Ride a short distance on day one and quit while you still feel good.
Your body will tell you what to do on day 2 etc.
IF you overdo it and have screaming sore muscles the next day, it's hard to climb back on the horse.
Your mileage WILL rapidly (way more than you expect) improve IF you keep it fun.