Sounds like the distance won't be a problem for you, if 20 isn't an issue. Just increyour the length of your rides gradually. 3250 feet of climbing in 50 miles is a decent amount though (not huge, but certainly challenging), so hills are what I'd focus on. Maybe start riding hillier routes and try and increase your climbing speeds and work your way to the smaller cogs. I think if you do that, you'll increase your distance endurance concurrently.