It may get 'boring' but trust me, if you do this a maximum of two times and see some drastic change in the mileage shown in the trip calculator, you will more attracted towards getting better numbers in the mileage calculator than overtaking everyone on the freeway.
Also, look at it like this: You can save 'thousands of rupees' by doing this.
I don't take hypermiling as 'fuel saving' ONLY. Rather, take it as fun. Shughal of getting the best mileage you can get by improving driving habits. If you consider the fact that speed 'wastes' fuel which can be used to travel more miles.
Also, by not driving at 120 and driving at 100 the time difference in your total commute will be of max 15 minutes. Its really psychological thing that you're going slow. 