Train D2835 Guilin to Guangzhou So. then C7655 to Zhongshan. Is same station can walk?

We want to travel from GL to Zhongsang. The train schedule shows GL to Guangzhou South D2835, then C7655 from Guangzhou South to Zhongshan North station. Are these two trains on the same station. How far is it? Do we have to go outside of the station to get off on train to get on to the other? Is it close enough to travel by foot or has to be by car, bus or taxi/ How safe is it to travel between stations? How far ahead can we purchase train tickets?