I do aquafit and although it is a great cardio muscle building program (and helps with joint mobility) it does zilch for osteoporosis so I am doing weights at least twice a week as well ...well that is my target (hopefully 3 times eventually). Probably 30 minutes walking would be adequate with those other sessions spattered throughout your week or if you can manage ..zumba is fabulous. ..lots of steps but it includes jumping which really helps with osteoporosis. With weights for spine health you'll need free weights /kettle bell type stuff not resistance. An exercise physiologist can plan a program. I've just started bodypump classes which apparently covers it all. Joining a fitness centre that offers all types of classes might be an option as you can keep changing what you do to avoid boredom.
I do mine on way home from work and always on weekends. With walking you could do shorter more frequent walks ..before work and in lunch break...it all adds up.
Not sure what they said...but I thought 150mins per week of exercise is adequate. Which is 30mins 5 days a week.
If you do classes like pump, zumba and aquafit it includes cardio with strength so unless you have lots of time on those days you wouldn't need to walk as well.
Just remember it's important to find something you like and the routine doesn't feel like punishment or it sure is hard to stick to.