Hey Duhfactor - Thanks! Gotta hand it to you on the name.
Two best areas for single track riding around St. George are: 1) Warner Valley (east side) and 2) past Navajo Drive in Bloomington (west side). Warner Valley is much more extensive although both areas offer multiple days of riding including single track and dirt roads (various quality). I ride from Navajo Drive more often as this is my backyard.
The ride out to Toroweap (and back) is an all day affair. Most of it is fast, smooth dirt. The last few miles are the most scenic, but all of it displays the vastness of the AZ Strip. The route directly south from St. George (River Road) via Mount Trumbull Schoolhouse is slightly longer, but better than the straight dirt roads from Colorado City or Kanab.
The Toroweap Overlook is perhaps the best for the entire Grand Canyon. Sheer cliffs drop to river level with views of water straight below.
Lots of rides listed on page 1.
Also a few $$$ for a Benchmark Map is well spent.
Late March should be a great time for riding around St. George. But, be cautious on the AZ Strip if recent rain (snow). Elevation is higher than St. George and some of the roads are really prone to sticky, slick mud. 60 miles is a long way back to pavement if your back wheel won't turn due to mud.