Give the trail at least a day to dry out after a winter rain - it can get sloppy in the first couple of miles. Watch for rocks on the trail after rains. It sees snow rarely, but ice is likely in the winter months.
From the Tucker Wildlife Sanctuary at the end of Modjeska Canyon Rd., head around the gate and start climbing. If you weren't awake yet, you'll be quickly as the first mile gains ~510 feet before a short .25 mile downhill. The downhill is over quickly and you start climbing again.
The Harding Truck Trail lives up to its name - it's hard. It's a truck trail / fire road the whole way up. Although wide, it does have more of a double or even singletrack feel than most OC Fireroads. One side will be more dominant than the other. Unlike Maple Springs Rd (the most common way to reach 4 Corners) you'll not experience vehicle traffic on Harding unless there is an event scheduled that day. It's a popular trail-running route. The trail will also stay hard-packed throughout the year and holds up against rain fairly well other than the first 2 miles. It's definitely more challenging though.
At the 7-mile mark you'll get a much-appreciated reprieve from the (average 8+%) climbing for about 3/4 of a mile before the last 1.5 mile charge up to 4 Corners. From 4 Corners you can head down to Maple Springs and road-ride back to your car, head up to Modjeska, Santiago Peaks, Joplin trail, Silverado Motorway, or simply back down Harding.
Coming back down is quick, with many water-bars available if you like to jump. Keep your speed in check for trail conditions, blind turns, and uphill riders and trail runners who will likely use the dominant track.