I hate to tell you this but Streets and Trips won't create a route on non-roads or wooded areas. On the other hand, Garmin's Mapsource software will allow you to place a "Way Point" anywhere on a map and you can have your GPS to lead you to that point. However, the software won't allow you to create a route out of these "Way Points" placed on wooded areas.

It's quite cumbersome not being able to create a route but this is your only option

Regards.