To expand a bit on jerry33's answer, when there are hills (even modest ones) it's more efficient to pretend you've got a car with a severely limited amount of power: watch the power meter and don't let the power consumption exceed some fixed value of your choosing, say for instance 40kW. If you do that, then on the uphills you will have to accept some slowing, maybe a lot of slowing if the hill is steep. On the downslope, don't let the speed exceed some fixed speed of your choosing, for grins let's call that number the 'speed limit'. Overall you'll achieve some average speed lower than the 'speed limit', but this technique will give you a small but significant improvement in efficiency over just setting the cruise control at that same speed.
It's also to your advantage never to allow the car to go into regen braking (i.e., power meter in the green), though if the hills are anything but gentle, you can't avoid regen entirely.
[edit: didn't see the '20kms steep uphill' comment before I posted this. It doesn't change the basic idea, though you may have to bump up the max power consumption you're willing to use in order to maintain a safe speed in the traffic conditions.]