Some observations, and ideas:
The number of waypoints most units can handle is far less than the number of track points (eg 500 waypoints for my SP3). Any route made from a track log needs to be "reduced" really.
If the track / route is along roads, you'd be better to recreate it by hand in MapSource I think, to gain the advantage of to auto routing. By this I mean - if you were to simply convert a track log to a route, the route would consist of many tiny sections, which the GPS unit would follow in "off road" mode, the only directions you'd get would be along the lines of "north to next waypoint".
I've just bought a program called Fugawi - it's a map viewer that can handle waypoints, track and routes. It comes with 1:50,000 OS maps, and can talk to all (they claim) Garmin GPS units.
For the case of a track log that doesn't follow roads in MapSource, what I can do is:
Import the track log into Fugawi (directly from the StreetPilot)
Reduce the number of points in the track (Fugawi does this automatically)
Convert track to a route
Send the route to the StreetPilot
Optionally then get the route from the SP to MapSource.
This creates an "off road" route which consists of a series of waypoints with meaningless names (eg R0018), but it's enough to catalogue and follow interesting green lanes and the like.
The only downside is that Fugawi, with OS maps for the whole UK, costs about £200. It is much cheaper than buying paper maps though.