Phang Na town to Krabi is about 80km by mostly busy highway. Take a bus like locals do - easy. Phang Nga is a very small town.
From Krabi bus station take a shared pickup (songthaew) to Ao Nang, about 20km then fixed-price longtail boat to Railay.
If you go to Phan Nga bus station in the morning, say 7-8am, you'll probably be in Krabi by 10, then transfer to Ao Nang and the quick longtail will probably take another 2 hours. On the sand in Railay by mid-afternoon at least