Hey - I was diagnosed a few years ago. Also on Adcal D3! I have enteropathic arthritis and mine was picked up on a routine xray of my hands & feet (done to check for deformity following a bad arthritis flare). I've had two dexa scans so far (one every 2 years). The second one showed no change in most areas, but a really bad, borderline osteoporosis section on my hip - but we believe this was caused by inflammation in the joint right by it (damaging the bone) so we're hopeful that it will recover.
I was pretty freaked out when I was diagnosed as my mum has it and my gran had osteoporosis so I know how awful it can be. But my lovely rheumatologist put me at ease a lot about it by talking to me about how they define the risks/results by age etc and explaining that while it sounded bad, my bones weren't actually all that thin.
Basically it's load bearing exercise you want, not just "impact" - so walking is good for your legs/hips (as your body weight counts as a load in that case) and I play squash & tennis (when the arthritis allows!) as that helps the right arm! Now my arthritis is under control I am pretty active; gardening, digging, DIY etc so my bones are getting plenty of work. I'm due my 3rd dexa scan next spring, but am pregnant so it will probably be delayed.
I don't know about you, but I really struggle with taking the calcium tablets, I have the citrus flavoured ones and they are just disgusting! During my 1st trimester when I was very queasy I simply couldn't get them down as they made me gag! I'm aware pregnancy is probably putting a strain on my bones so I'm making a big effort to try and remember to take them though, plus eating plenty of calcuim rich foods too!
You don't say in your OP how old you are - but have you discussed with your doctors why you have it? Is it an age issue? Diet related? Understanding why I had got it (probably partly genetics, but mainly due to medication I'd been on for 10 odd years for my arthritis) helped me focus on how to manage it rather than just freaking out about the future! I was in my late 20s when I was diagnosed so it was a huge shock for me and I was really worried about how bad my bones would be by the time I was 40, or 50 or even 60.
Oh - and folates are important too - taking a folic acid supplement / eating a diet rich in folates might help you recover some density (your body needs folates to rebuild bone). Perhaps ask for a referral to a dietician for help checking over your diet and making sure you're getting enough of what you need might be a good idea!
I'm not expert on it - but if you have any questions do ask and I'll do my best to answer! :)