I've only ridden the towpath between Georgetown and Harpers Ferry, and haven't been on it much in the past few years. But, here's a good place to start planning:

http://bikewashington.org/canal/index.php

Also there's a COTowpath yahoo group with many people who have ridden the whole towpath/GAP and can give lots of advice and details on stores and places to stay etc etc.

BTW the weather here in late April is pretty varied. Could be 80 and sunny, could be 40 and rainy or overcast.