That's not interval training that you're doing. It's just improving your fitness enough until the point when you can run without walking. Interval training where you walk or stand as a rest between intervals would involve running fast during the intervals.
1. If you have a track nearby, then that's a great way to measure intervals. If you have a stopwatch, then you can measure your times too, but as a beginner you can improve a lot without worrying about specific times and just go by the feel of the effort.
2. A GPS watch is a nice and convenient (and accurate enough) way to measure time and distance almost anywhere that you will go to run.
3. Go out on the roads that you run on and measure some distances (100m, 400m, 1k, 1mi., whatever you want). You can use a car or anything to measure. Again, the exact accuracy isn't very important for your training. You can just measure relative improvement over time.
4. Go out and make some marks on the road or use landmarks like light posts to run consistent distances. It doesn't matter how far the distances actually are, and you could approximate all of the standard interval distances decently well.
5. Don't measure distance and just measure time. Run as hard as you want for prescribed times.
Think about it. If you are consistent and disciplined about your training, do you think that your legs will really know the difference whether you are running 350m intervals or 450m intervals? I bet you can measure more accurately than that anyway.