Terry, I don't use an HRM, but I can pretty much guarantee you that if you try to stay at 130 bpm riding that course, it will take you an hour and a half to go around once...    but then again, you're doing 24 hours, so it's maybe not a bad idea...
 
The real keys to surviving this 20 mile loop are  1) don't burn yourself out on the first 8 miles of climbs;  2) Figure out which hills on the back side you can carry most of your speed up, and which you can't; and 3) realizing that once you're past the 15 mile point, it's mostly downhill from there, except for the very last climb before the Goatneck Hill descent, and the last 100 yards of incline to the finish.
 
Good luck, we'll be rooting you on...  you've picked a tough event!
 
A word of advice -- bring some warm clothing, it gets chilly at night, between laps when you're sweaty...
 

Im really liking this Garmin Forerunner 50.  I purchased it for only $46.89 (shipping included) off ebay.  
 
It uses the same wireless components as the high end models so I will be able to upgrade latter to the Edge 705.    And maybe even get a Power Tap hub!