The mountain is covered in firetrails, most of which are too gentle to be useful. The good stuff is on the north side: steep, rocky little trails that are a hazard when descending and an endless taunt when ascending.
Ascend the Back Creek trail until it ends at Murchio Gap. Cross the firetrail there to continue uphill on the Bald Ridge trail; follow Bald Ridge to Prospector's Gap. Go west on the North peak trail to Devil's Elbow, then north on the Summit trail to the top. Get water here, then return to car. You've gone 11.0 miles with 4000' of climb and 4000' descent. A typical Hardrock climb, minus the altitude. Repeat 2-3 times for a nice workout. If you manage 3 mph and do well at altitude, you may be ready for Hardrock.
Other good routes are Mt. Olympia and Eagle Peak. In the wet season the Falls loop is interesting. It's also possible to go cross-country on many parts of the mountain, just to make things more difficult, but watch for poison oak.
If you're totally immune to poison oak and ready for the most hellish climb on the mountain, try repeats of the East trail on the far flank of Mt. Olympia (climbs 2000' in one mile on unmaintained trail). I sure wish I were immune, cuz then I'd be able to train on crap like this.
From Regency Gate, take the Donner Canyon firetrail to Cardinet Oaks Road and turn left. Once across the stream and up a couple switchbacks, look for a good place to leave the road and start climbing straight up the grass (nearly 100% grade). Stay to the highest part of the ridge. By keeping to the grass, you can avoid the poison oak. Cross the Mt. Olympia road, and proceed directly to the Mt. Olympia summit.
Go south on the North Peak trail as it dips into a saddle. The trail then climbs through some scrubby poison oak. Stay on the trail until the poison oak thins out and you near some scraggly oak trees, then look for a good spot to leave the trail and attack the rock escarpment.
Climb the spine of the rocks (Class 3) all the way to the North Peak transmission tower. There are a few places where an easier route (Class 2+) involves traversing a bit beneath the spine.