Heat map setting when Creating Route

I want to create a route for a Run that involves some trail running sections that are not mapped but show up on the heat map.  When I go to create map the heat map disappears.  Why is this?  Why does a heatmap need to be sport specific.  Most Trails are multi use and there doesn’t seem to be an option to show all heatmap information combined with the ability to create route.  It would be even better if you could create a route to follow the heatmap.