You would be better off walking it. There is a mad rush to the monorail after the fireworks. We made that mistake once as we were closer to the monorail than the front of the park and the whole time we were waiting (probably between 30 and 45 minutes), my DH kept looking at me and saying over and over "We would be back at the Disneyland Hotel by now if we had just walked".
You will have no problem at all getting there in time if you walk it. It's a 10 minute walk.
I agree, sometimes the wait can be long at the end of the night. I'm also not sure how late it runs. The walk through Downtown Disney is nice. It takes us about 15 minutes.