I'm running a play by post game set in Icewind Dale. I would like to know how long it should take to go a given distance, or from one town to another, assuming no special obstacles. Is there a handy travel guide for this?

I'm not sure there is one. Icewind Dale and the towns traveled to are not even in the official Faerun map.

Check this map from the Forgotten Realms wiki:
http://forgottenrealms.wikia.com/wiki/Ten_Towns?file=Ten_Towns.jpg

It shows some of the major towns and landmarks in the area, and provides a very rough scale at the bottom. There's also a more detailed map at the top of the article, but without any distance scale. However, you should be able to compare the maps and swag an estimate between the two.

Icewind Dale (the game) includes Kuldahar, Vale of Shadows, Dorns Deep, Wyrms Tooth and Severed Hand, all of which are east and south of Easthaven and off that map.

What you can do is load a save where you can travel to almost anywhere. Then have your party travel do different locations. Look at the time difference and record it.

Also, there are no "special obstacles" when traveling between maps. I'm assuming you mean story events and random encounters, like in Baldur's Gate. But *within* particular maps some enemies will re-spawn.