To be successful in weight loss do I need to have defined goals?

I want to lose weight but not to be a specific size/weight, it is more about health and getting to a point where it's manageable.

I'm reluctant to set a weight/size goal because I don't know what is going to be manageable/realistic and I don't want it to be about being disappointed if I don't reach or maintain that number.

I also need ideas on the lowest impact possible exercise, we already have a recumbent exercise bike but I'm already finding it very boring and need more options to stay interested.

I've had 2 crushed vertebrae in my spine for many years (T7 & T9) and last year found out I've also done damage to another 2 (T8 & T10). Due to this is it worth joining a gym or will it be a waste considering most exercise options there would be high impact?

Thanks for any advice you might be able to give!