Without looking at my notes, and bearing in mind that new highways may have been built, I think that even with a car you'll need about three hours to get to Yongding. The best route to Hukeng, the site of the best-known group of buildings, is not via Yongding but by direct bus or sometimes changing to minibus at Longyan. The quickest route by car would be the same.
You may possibly find it difficult to find a cab in Xiamen willing to venture out that far or knowing the route, but it's certainly worth a try, stopping a few drivers the day before. It's about 200km each way, and well worth the trip. Some of the buildings that are not officially open, such as the Huanxing Lou, are well worth wandering into as well (asking permission from one of the few remaining residents). This one has damage to one section that dates back to the Taiping Rebellion, and its original stone beams to fortify the doors still intact.