I think this question is a little more complicated than it appears at first pass. I will demonstrate by answering two forms of the question.
Q1: "Is it possible that, despite not having regular training, I would be able complete a 50km without it being a traumatic event?"
To this question, I say absolutely! Riding on fairly flat pavement at low speeds is not too difficult, so as long as you pace yourself properly, this is totally manageable. Never try to push yourself too hard and stop for snacks every hour and it may even be fairly enjoyable.
Q2: "Is it possible that riding 50km, not having regular training, would be extremely miserable?"
Also yes! If you don't ride often, there are a lot of potentially very unpleasant things that could happen on such a ride. This includes:
(1) Extreme fatigue. If you don't ride regularly, it can be very easy miss-pace yourself. It might not feel like your pushing that hard at 0-10km, but suddenly you can find yourself having trouble keeping the same pace around 15km. That's a real problem when you have 35km more to go!
(2) Very difficult terrain. Riding on flat pavement at a relaxing rate is not so bad. Going up hill at any rate can be pretty taxing, especially if you don't ride much. In my opinion, going over sand is considerable harder than riding up hill. If even 1km of that ride is over sand, for many reasons, I can pretty much guarantee it will not be a pleasant experience. I highly recommend not riding over sand in your first ride. Actually, I highly recommend never riding over sand ever; see next issue. Gravel is somewhere inbetween concrete and sand in terms of physical effort required, also raises probability of sliding out and getting hurt.
(3) Mechanical breakdowns. There's a chance something might happen to your bike. You might get a flat tire, the chain might break, or you might ride it through sand, destroying just about every moving part of the bike. If your bike is well maintained, the probability of this happening on any single 50 km ride is low (assuming you avoid sand), but the probability of this happening on one of 20 such rides is pretty high. For a bike sitting in a garage for years, the probability of this happening on the first 50 km ride is actually quite high.
(4) General discomfort. If you haven't been riding for awhile, getting used to a saddle can be...uncomfortable. Not a big deal is you're going 5km, but really starts adding up around 20km, meaning potentially completely unbearable around 30km, and you've still got 20km to go.
In summary, a 50km ride without practice may go really well and be fun. But there's a lot of potentials for it not to, especially if you're not just a bike nut who hasn't ridden in awhile. As such, I personally would recommend giving it a try, but with a solid backup plan. For example, maybe there's a bus route you can ride along (make sure they allow bikes!), so in case any of those issues come up, you can easily bail out.
Also, do not ride over sand and I would advise avoiding gravel. Finally, get your bike checked at the local bike shop.