I live 30 minutes from Magic Mountain and an hour from Disneyland just off the 5 freeway. It will NOT take you an hour to go from one to the other--not unless you drive at the dead of night and drive 75+ MPH. If you drive during the day or early evening, expect at least 90 minutes to get from place to place.
If you opt for rush hour 7:00-9:30am and 3:00-7:00pm, it will take you at least 2 if not 3 hours to get from Valencia to Anaheim on the I-5. That freeway is a total parking lot during rush hour--just too many vehicles on the road.
I say go down the night before and stay in a motel near DL or plan to get up early and be on the road by 6:00am. You'll get to DL extra early, but there are plenty of things to do. Downtown Disney isn't really open until 9:00am-ish except for a few restaurants and the World of Disney mega store. You might be better off going to the Grand Californian or the Disneyland Hotel for breakfast and a bit of window shopping. I highly recommend breakfast at Storytellers in the Grand Californian. A little pricey, but nice setting. Plus it's a character meal of sorts--Chip & Dale and the Brer critters are there. I think Mickey in camping gear also shows up--can't remember.
Definately make the trip down if you can, but don't expect to travel at any rapid pace on the I-5 unless you're driving way early or in the middle of the night.
And if you're adventurous (and have a good map), I highly recommend a round-about yet much better moving route--take the 5 south to the 210 east (it's about 10 minutes south of Valencia), go on 210 for a looong time til it becomes the 57 (right around the I-10 interchange). From there, the 57 will take you down to Anaheim. Exit Ball Road. You'll be right at Edison Field (home of the World Series Champion Angels) and at the Pond Ice Arena. If you pass the stadium, you missed the exit. Head west on Ball Road for a few miles (like 2 or 3 total) and you'll run right into Disneyland. We do this route during rush hour and get to DL in about an hour. Just be sure to get a good map.
