I enjoy cycling, it's always been something that has been a part of my life.
I recommend it as a form of exercise and relaxation all at the same time.
It's very important to get the right size, fit and adjustment, as Egon and Aky217 have stated.
12 miles is a long way if your not comfortable. You will find that there may be some sore spots in the beginning but after a few trips and proper equipment it should become quite pleasant and I suspect if you do your home work (just like you are now) you won't regret this endeavor. :thumbsup:
There is a lot of hype surrounding the sport or hobby. I was all caught up in it years ago. I try not allow my self to get enamored with the latest trends....unless it works. I try to keep it simple now.
At your size, it's likely that the largest frame would not be "too large". I'm slightly shorter than you, and l like a 60mm frame, on a Giant, that's a large. (I think giant use only 3 frame sizes? S, M, L).
How come I need fenders? I'm a fair weather rider only.
You don't need fenders, I recommend passing on the fenders.
The bike I'm looking at has a front shock absorber suspension. Do I need this and is it a big deal?
I personally would pass on the suspension, but that is something you might want. It can be added or removed by replacing the fork later or added later, but it's a bit pricey.
I would recommend you go with a thread-less headset to made customizing easier.
Question...Can I get a cardio exercise out of this?
It depends on how you ride, but it's a lot more cardio than sitting on the couch :laughing:.
I think of the bike trans sort of like my tractor trans. I pick one of the three ranges I need to operate in depending on the terrain. ( I ride off road as well as on road....with the same bike.) It's important to try to anticipate a coming obstacle and understand that you need to shift into the right gear before you reach it and understand that your transmission only works while pedaling. After a while, you will shift without being conscious of it.
I think of my legs as the engine, and the engine never stops....as long as the bike is moving.