Measuring it by the distance makes no sense. Endurance is a relatively low power. If you ride on a flat, you will be moving relatively fast, but if you get on a hill, you might be riding at 6-7 km/h… So you may do a long workout but might move only 15 or 20 km.
Endurance rides are usually long and most of us may not have time for them mid-week. When I do endurance rides, I try to do them on weekends. It is better to set a goal of doing several such rides per week. For mid-week, I leave shorter but more intense workouts.
Multiweek programs for Century or Grand Fondo should have endurance rides. If you won’t find one, feel free to send me a PM, I can e-mail you the endurance workout which I created. It scales for one’s FTP, so it will be mid of endurance zone for you, too. But it is really a super simple workout - the same fixed power for hours 
Sometimes endurance workouts change the power level by 10-15 watts up and down every 4-10 minutes, to make it less boring. Not sure if that helps, though.