I drive from San Diego to DL fairly regularly. Rush hour traffic going north really isn't that bad - it's afternoon that north is bad. Leaving my house (which is a few miles north of Mission Valley) at 7:00 I can be at the turnstiles by 9:00 (after parking and riding the tram) usually with no problem. From downtown/Mission Valley you'd probably be fine leaving by 6:45 (that means sitting in your car at 6:45, not leaving your room at 6:45!)
One thing I would recommend is NOT taking the Disney Way exit off of I5 - you can spend a lot of time making your way to parking if you do that, depending on where they make you park.
Go past Disney Way and exit on Harbor Blvd. Go right at the top of the exit ramp, and then get in the left lane to turn left at the next stop light onto Ball Road. From there, you're going to turn left onto Disneyland Drive (I think it's the second stop light), and then follow the Theme Park signs - you'll park in the Mickey and Friends parking structure. From there take the tram to the park entrance.