Trying out a new training program called the 180 forumua. It focuses on your aerobic ability by training your body to use fat as its primary energy source rather than sugar.
You can read a number of articles written by the creator of the plan (Phil Maffetone) here..
Here is a general overview of the plan and how to calculate the heart rate zone you need to stay in
For me - I need to try and keep my heart rate in between 130 and 139. Since it usually hits 160-170+ in any given workout - it was much harder than you would think. HR gets to high - shift gears, then it drops to low - shift gears... Lots of shifting and a bouncing HR. I actually did ok once I got the hang of it, but it is much more difficult than you would think.
Still a great night for a ride (cool, low wind, low traffic, not many bugs (yet))