Half book time is pretty damn fast too. I fast walk when I can, so I gotta figure you're at jogging speed by that point?
Book time on flat ground is 2mph, a pretty easy walk. To do it in half the time you need to average 4mph (15-minute miles) which is a brisk walk or a pretty slow jog. You can quick-walk and not take breaks, or walk some and jog some. When there's a goodly slope (1000ft/mile), book time is 1mph, a really slow walk. To double that pace, just keep moving steadily uphill at your normal walking pace (the effort level will be higher, but the rhythm of your steps should be the same - you're not jogging, you keep a foot on the ground at all times), and don't stop for any reason. You can gain extra time by jogging on the flatter sections - you'll still experience them as restful compared to the uphills.