1. Bring a stroller.
2. Do not push it. If your child needs a break, take a break. If you don't, your day and possibly your trip will be a disaster.
a. Take a midday break
b. Play it all by ear. Disneyland is amazing place to me because of how joyful the unplanned moments are. The time we went as a family our then two and three year old loved the carosel. We rode it many, many times.
3. Get a babysitter. We've used destination sitters and couldn't have been more pleased with the exception that our then four-year-old wanted to play Sorry for the next three months.
See the trip through your child's eyes. The smiles, oohs, and ahhs are great.
Addendum to #2. remember that you brought your child. Their wellbeing trump's your desires/urges. If theyre on the verge, but you only have this one chance to ride pirates. You tell them yes let's take a break.