The weather in late November might not be conducive to spending 4+ hours walking in the woods, so I agree that waiting is a very good strategy. If the weather's poor and you have already had enough time to do the key sights in Zagreb, you could take the train or bus to Ljubljana for the day. Round-trip will be 4 or 5 hours, but you'll be in a city where you can easily pop in and out of restaurants, shops and museums if you need a break from the fall weather.
I am influenced by having had a pretty miserable day in Zagreb on October 11, 2015. It poured rain all day and was quite chilly. It was a holiday and nothing much was open, so I basically holed up in my hotel room. I can't imagine enjoying Plitvice under those conditions, and you will be traveling 6 weeks later than I did.
If you're lucky with the weather and can't find a tour, taking a bus will probably be workable. Just check carefully about return buses and don't plan to take the last one back to Zagreb. In this situation (very off-season, going to a rural location), I'd recommend getting an early start so you have maximum options for the return trip.