If you can see my signature, you'll see I've done a LOT of WDW races, so I promise I speak from experience! I always start with my intervals - the only exception being in very cold weather I'll walk for a good 5 minutes before any running, just to get my muscles warmed up so I don't strain anything that's cold and tight. Throughout the race, not just at the start, there will be times when you'll need to run a little longer, or walk a little longer, just to go with the flow in tight quarters: don't worry about it - roll with it and pick your intervals back up when conditions permit. This never lasts longer than a few seconds to a minute.
Some general tips for everyone, runners, run-walkers, and walkers, alike:
1. Be aware of your surroundings! Just like with driving a car, no sudden moves - be sure to look for clearance before moving out of your current path and wait until you do have room to do so safely.
2. When approaching someone/s moving slower ahead of you, don't get so close you're nearly on their heels! Slow down if you need to, and pass when you have an opening. A friendly, "Excuse me - may I please come through?" can help if you're completely blocked by 5 people walking abreast. Be nice!
3. Raising your hand to signal a walk is great, but you need to do so
before you start slowing down! People behind you assume you'll continue moving at your current speed, so the raised hand signals, "Yoo-hoo, I'm going to do something different in a sec, so please be aware!"
4. Try hard to leave at least a single-file lane open for anyone faster to pass. On the highways, there's lots of room to spread out, which makes it easy. But in narrow spots, avoid having multiple people side by side at the same speed. This may mean running a little longer until you can safely get behind another person walking, or walking a little longer until you can safely get into the running lane. The dynamics of a moving crowd are fluid and lanes will open and close randomly - this is why #1 is so important.
5. PLEASE do not cut across the entire width of the course because it's time for a walk interval (or for a photo, or for a restroom, or for any other reason)! So not safe. Again, look around, see where is the safest close place to slow down - sometimes, that's right where you are!