You can also drive straight up I-Drive if you like. I found that the lower section of it isn't too crazy with traffic northbound in the early morning, as most of the properties on it cater to tourists, and most of them are going toward WDW, and are not out and about in the morning hours like commuters. IME, the nasty part of the I-drive traffic really doesn't come until a block or two north of the OCCC.
The convention center is just a couple of blocks north of the intersection of I-Drive and the Beeline, one exit off of I-4 if you take that route. Don't make the mistake of trying to go the "back way" up Apopka-Vineland & Turkey Lake Rd., then across to I-Drive via Sand Lake Rd. You'll sit on the part of Sand Lake Rd. between I-4 and I-Drive forever, then have to battle your way down a congested section of I-Drive to the OCCC. I tried this last summer--bad idea. I-Drive northbound all the way is a longer route, but I found that it had less traffic than the other two routes, once you actually get out of WDW and onto I-Drive northbound. To avoid the worst I-Drive traffic, I make it a rule never to drive any further north on I-Drive than the OCCC, and I try to avoid the I-Drive/Sand Lake Rd. intersection at all costs.