That depends on how you approach it.
If you only do it once a year, like it's customary in my region on Father's day [1], the seriousness lies more in getting home safely despite being drunk, rather than in the actual cycling part.

But if you do it on a regular basis (while staying sober I might add), 50km is a serious distance in terms of training effect. As a rule of thumb it is often said that you need to cycle for at least 90 minutes at a stretch to get a long-term training effect, because at that point you'll have burned through your excess carbohydrates and your metabolism will rely entirely on burning fat (see "basic endurance").
It doesn't really matter at what speed you ride, as long as you ride continuously, but when riding 50km in 90 minutes you'll have an average speed of ~33km/h, which even with a road bike on a relatively flat track isn't that easy.

If you use a different bicycle or a more challenging track you'll need even longer and burn more energy.
For example, have a look at Koiruus's picture! A Fatbike on a hilly snowy forest path will require much more energy than a road bike on a flat and even road track. If that track is only 25km long and looks like this all the way, I'd consider it challenging to finish it within 90 minutes.

Oh, and of course it's never too late to pick up cycling (again)!


[1] https://en.wikipedia.org/wiki/Father%27s_Day#Germany