I find that having a full trail map on the receiver is helpful, but not truly vital.
First, I mark a series of significant points along the trail on a paper map, each with a unique ID. These include, depending on the hike, trail junctions, major turns, USFS gates, beginnings/ends of sharp ascents/descents, peaks, saddles, stream crossings etc. If the trail has a long segment with no major feature, I pick a point or two along that stretch too.
Then I open the GeoPDF that covers the area in Acrobat Reader. GeoPDFs are scanned (raster) and geo-referenced 24K quad maps free to download from store.usgs.gov.They are used with a free GeoPDF toolbar, available from www.terragotech.com
. On the GeoPDF I set the same parameters as my MapSource (WGS84 and lat/lon as decimal degrees) and then, holding the paper map next to the screen, mark my waypoints. This gives their coordinates, which I then plug into MapSource and upload to the receiver. It sounds like a lot of work but isn't because there are usually c. 7-20 waypoints per hike. And it goes quickly once you start.
Note: with digital maps you don't have to use UTM, just make sure that all products you use are set to the same datum/coordinate system. UTM is only important for reading coordinates off paper maps (in this context). Google Earth, by the way, uses WGS84.
If you have a markable, receiver-usable 24K digital map, you can skip the GeoPDF step altogether and mark waypoints there directly. (Garmin's statewide 24K sets come on chips and are not [yet] markable on the computer. The National Parks & vicinity series is).
Thus, while hiking, the actual trail is not visible on the receiver but the key waypoints are. Since they are also marked on the paper map you can tell exactly where you are (beware of hiking without paper topos). In short, you don't really need the whole trail on the map, only key points along the trail. I hope this helps.