I will visit Hunan for 8 days.

It seems so. You actually can have a better trip by staying more time in other places. Three whole days in Zhangjiajie are enough to cover almost all must-sees. If you don't mind, I'd make some changes of your plan to fulfill the trip:
Day 1: Changsha - Zhangjiajie
Day 2-3: Zhangjiajie
Day 4: Zhangjiajie - Fenghuang Ancient Town
Day 5-6: Fenghuang Ancient Town
Day 7-8: Changsha downtown sightseeing and visit

Hi Michael,
I have some suggestions on your itinerary.
Actually spend 8 days the province you can visit more scenery within this time. If u don't mind I 've below suggestion based on Irina.
Day 5 to Day 7 Hengshan Mountain
Day 7 to Day 8 Hengshan Mountain to Changsha
(there is high speed railway from Changsha to Hengshan west station with 0.5H, Hengshan mountain is one of the famous 5 mountains in China, it worth you to go there and have fun).
if you need more details can touch with me, Joy liu, my mail: joyliu188sina.com