I was bitten by this just the other weekend. Out in the middle of nowhere for an organised event, and the organisers said we had to use the backup route. I had it downloaded to my phone, but Garmin Connect won't work at all without an internet connection.
Ended up just using my phone for navigation, and even though I was recording GPS and heart rate the whole time, I'd only used 50% battery after 14 hours riding. Nice.
There was another guy at the campsite who was looking for someone who had downloaded the route. I AirDropped it to him, and he transferred it to his Wahoo bike computer with no trouble.
For shorter rides/runs, I use the WorkOutdoors app on the Apple Watch. It's only battery life that's an issue when the activity is longer than 6 hours. Just this morning I had the opportunity to go for a run a bit out of my normal zones, so within a minute I had sketched out a route on the phone and transferred it over to the watch. (That some of the paths I'd used didn't actually exist anymore was a separate issue, but I've already updated OpenStreetMap to fix that.)