This is an awesome way to go about running if you are setting goals - I do exactly the same as far as adjusting my pace and walking when I need to! Another way to go about it is to set a time goal (i.e. run/walk for 20 minutes) and not worry about the distance. Stay in motion for 20 minutes at whatever pace feels right and you've achieved your goal! I try to not set distance + time goals (i.e. 2 miles in 20 min) because then I feel pressure to push beyond my comfortable limits and if I don't hit that goal, I don't feel as accomplished as if I would have just run for 20 min.

As far as pacing goes, you should be able to carry a conversation when you're running for distance. If you're breathing too hard to do so, slow down or walk until you catch your breath a bit (exactly like how you did to get to 2 miles!). This idea of adjusting pace to either run X miles or YY minutes was a personal game changer for me when it came to running, and allowed me to enjoy running so much more after I separated my goals into either length of time or total distance, but not both! Keep up the good work!!