This is a one-way trip to Huntington Beach from the Valero station at the corner of Apple Valley Road and Highway 18 East.
Road conditions
With the exception of the bridge over the Mojave River on Highway 18 East, the roads are not an issue. True, you do ride on I-15 between Highway 138 and Cleghorn Road, but the distance isn't that far, and you're going downhill. Once you get to the Santa Ana River Bike Trail (SART), it should be smooth sailing.
Maps
Variations
To avoid the bridge on Highway 18 East and the trip down Hesperia Road, you can go down the bike lane on Apple Valley Road, turn west on Bear Valley Road, turn south on Jacaranda Avenue (which turns into E Avenue, and then jog over to Summit Valley Road, at which point you follow the map above.
Once you get down the hill, instead of turning on Glen Helen Parkway you can stay on Cajon Blvd, go down to North Mt Vernon Avenue, cross the I-10 to South Mt Vernon Avenue, and then pick up the SART there.