I am just starting to train for my 1st marathon. I am going to be doing most of my training on a treadmill. For getting started, should I focus more on increasing the length of time I run/walk overall and not worry about how many miles I get in or should I focus on getting to a certain number of miles regardless of how long it takes?

What worked for me when I was younger was run 10 miles in the morning and 10 miles in the evening, I ran everywhere, loved running, it helps if you have learned to enjoy it or the benefits.
It will greatly help. Find the most difficult, up and down stadium stairs, run backwards, anything to make it tougher.
I used to run to the base of the mountains in southern Nevada and look back at the City an see the distance travelled.
Pretty cool, I used to think of Mercury(messenger of the Gods), and thought it was neat how far a person could travel, and not worry about a flat tire, mechanical breakdown or whatever, I felt it was 100% sure way to travel at the time, I had been lucky with no blisters, sprains or whatever. Now running it not that reliable for Me, anything could happen.
A buddy of mine and myself ran from Angel Peak, down across the snow patched mountains, to Vegas, across Vegas Valley and to the Base of Sunrise/Frenchman Mountains 40 Miles in 10 hours. Seems like a lot, but averages only 4 MPH?
Basically I would say do your running outside, with the smells, the heat, everything.
A Treadmill and HBO sounds nice. It can't be that easy.
No Pain, No Gain.
Running helps your Endurance, you think clearer, You cool off faster and regain your breathing and heartbeat faster after exercise, just good stuff, just a little tough, and requires determination.
Oh, yeah, do sprints as well.
I used to ride a Unicycle and wheelie bikes at the time, both of these require more than your normal exertion or you will fall, start over, hurt yourself or something where a momentary burst is required, I think it helps.
Good Luck.

My breathing was out, out, in, in, repeat, for each step, and becomes automatic, and needs to be automatic.