Wow, that's quite heavy! 143 lb weight loss is phenomenal. Going from 12.5 to 25 miles by September is totally doable. Hopefully it will be a fun journey.

In your case I think having an excellent bike fitter adjust your bike and physical therapy are critical. The bike fitting will make sure that your bike works for you and your body so that you can be most efficient and, most importantly, not hurt yourself. A PT is the best person to turn to for exercise help, because they will evaluate your body and based on that and experience can tell you what to do and not to do for your specific issues.

I wouldn't discount the shoulder or heart issues as not going to be a problem, either. Riding can stress your shoulders to a varying degree depending on bike fit and posture, and the different angle your arm will be at than normal could stress it.

I have a minor issue in comparison, but without PT and a bike fitting I would not have successfully trained up to where I am (or was, considering I've been sitting on my butt for months due to weather). It took me five years to finally go to physical therapy and get a proper diagnosis, and the drastic reduction in chronic pain was amazing. I still go occasionally; I was having a hard time walking this winter and had to go back. I got exercises to do to build up the muscles around my SI joints and got my leg pulled back into position.

BTW, my bike fitting fell under the category of PT with my insurance company.

Good luck!