How to your start a new habit?

At my physical a few month back I had gained about 10 lbs. It didn't come as a surprise. Some pants I could previously fit comfortably weren't any longer. Unfortunately, I'm not naturally thin and I tend carry excess weight in my middle section. After 4 c-sections, my stomach is paunchy and not a whole lot to look at. However...this body gave me 4 fantastic kids so I try not to hate on it too bad. I'm trying to get the rhythm together to start run/walking again. It's cheap...no real equipment needed, burns tons of calories and you can do it most anywhere. Now the hard part, getting a routine going. I live kind of in the sticks so I may have to run on my own. I thought I'd join a group, but that hasn't worked out with my work schedule. I think the reality is I'm just going to have to bite bullet and start moving.
So, I've chosen to slooowwwly adopt a few new habits. I've started walking with a couple ladies from work 4-5 days/week...we walk for 30-45 mins. It feels great to get moving and eliminates the excuse of not having the time to exercise. I've also been trying to drink more water. Something I'm not very good about.
The hardest part is how to sustain a new habit. I've been thinking about it a lot. Especially one your work out buddy dropout.
 I need to be do this for me, regardless.
One step at a time, one day at a time.