I've been doing a good little loop recently, when I've not had time to go on longer rides (or it's been hot all day and I've wimped out till evening). It's about 17k's with 380m of climbing, and I'm doing it in about 45 minutes (4, 5 times now), so 23-24 k's an hour.  What's the best way to pick up the pace, given time constraints?   Cheers