Yeah Scrap is right spin classes are a better option. Cycling on a road or path it can be difficult to keep your heart rate up because you'll find you need to stop and start a lot for lights and obstacles etc.

If you're busy the time required to upkeep a bicycle in really good order can be annoying too.

The thing with cycling is that 12 miles is not really that far unless you can do it very fast and that depends on traffic etc.

I don't think you are as likely to cramp up cycling as you are running but I would warn against carrying anything on your back and watch your posture. Also You get large quads from cycling which is fine but you'll need to remember your hamstrings in you weights or resistance workouts to make sure you balance them and stretch.

I'm thinking spin classes would be better for you for climate reasons too - cycling in the rain is not the funnest thing in the world!