How do I change the default location?
If it is possible will it affect my bookmarked routes?
To change default location, follow the "profile" link in the top bar. (If you don't see it, note that it's only present when you're logged in.) Once there you'll find a "Set Default Location" widget. Click anywhere on the map to drop a pin, and that is your default location.
The default location does not affect any saved bookmarks. Once you've set it, any visit to a blank Main Map page will result in a marker at the default location. You'll be able to choose whether to start your route from the default location, or start from somewhere else.