Add some hills AND increase your speed. Or, increase your speed first, then add some hills. Or add some hills, then increase your speed. The thing with weight loss is that you can never get fully comfortable (yes, it SUCKS) but it's like the C25K (I think it's a great program), it bumps up your training every week because your body IS adaptable.
As for zig zagging, get comfortable monitoring your calories first
I would suggest to not have "cheat" days in the beginning because you need to get in the habit of recording everything. Zig zagging works like this though: you might have 2 high calorie days, then 2 low ones, then 2 medium ones. Or one high, 3 low, one medium. This way your body doesn't get "comfortable" in a routine.
And, if you're ONLY doing cardio, I would recommend some weight lifting/strength training. Cardio only burns calories upto about 4 hours after you do it. Muscle burns calories all day (which is why men lose weight faster because they have more lean muscle mass, damn them).