50k is not a short ride.

try doing some shorter rides, 15, 25, 30k and then go back and do a 50k again.
What has helped me the most is frequency of rides.

Last winter, for the first time, i did some spinning on a wind trainer. Unlike some people, i find it difficult to sit on that thing for more than 30 minutes. But even 30 minutes once a week made it so much easier to get on the bike and go once the weather permitted.
Just one day you're going to do your 25k ride and think, huh. I'm not as tired as i thought i'd be.

Schedule a bigger ride. ride with others. That helps too. Is there any one thing
that is limiting your ride time?
(after 40k your feet hurt, your back, etc) try to work through the problem by adjusting your bike, changing your diet, and then get on there and try again.
I think you're well on your way.
stay cool.
Mimi in Seattle, the cool capital of the country today.