Oh, one more thing, don't forget to factor in the time for climb/descent. It's not a 20 minute jolly. I reckon it will take around 15-20 minutes to get up there and 20 minutes back down again. 20 minutes in the cruise and you've got an hour logged.
Don't forget to consider the winds aloft as well. They can be quite strong at those levels (50-80kts) when it's fairly benign on the ground. With a groundspeed of 20 knots into wind, it will take you a long time to get somewhere!