As I understood, you need to get a path length between several locations.
It's not possible in MapField, you can only set that route by yourself.
Invoke BB Maps for route direction
You can always use MapField in you application for simple functionality, like present locations, and use BlackBerry Maps application for enhanced stuff like select locations and route directions presentation for user.
To invoke BB Maps for route direction use code:
Invoke.invokeApplication(Invoke.APP_TYPE_MAPS,
new MapsArguments(
MapsArguments.ARG_LOCATION_DOCUMENT,document));
With document formed like:
<location-document>
<GetRoute>
<location lon='-8030000' lat='4326000' label='Kitchener, ON' description='Kitchener, Ontario, Canada' />
<location lon='-7569792' lat='4542349' label='Ottawa, ON' description='Ottawa, Ontario, Canada' />
</GetRoute>
</location-document>
Result will be like
alt text http://www.blackberry.com/knowledgecentersupport/kmsupport/supportknowledgebase/images/DB-00599_3.jpgalt text http://www.blackberry.com/knowledgecentersupport/kmsupport/supportknowledgebase/images/DB-00599_4.jpg
Now, in BlackBerry Maps you can register your App menu item and start or update your App with selected location using MapView.
See How To - Add an ApplicationMenuItem to BlackBerry Maps
UPDATE
Use the gmap navigation service for driving directions
See J2ME/Android/BlackBerry - driving directions, route between two locations
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