You might want to throw in a bit of hill climbing. 60k on the level and 60k of hills is very different. The hills require some technique in picking up speed on the downhill, maintaining the speed through the bottom, and then slowely reducing to lower & lower gears as required on the uphill.
Have fun. Remember it is not a race. Each day is broken up into roughly four 20k to 25k rides (if you stop at each rest point). Try building that into your training.