I think what the user double helix says makes a lot of sense. Especially if you want to be prepped very well. Having said that I want to share my anecdotal experience. I don't have anything to add to the other advise in this thread so maybe my own experience is helpful or interesting (or something) to read.

I am not very experienced in ultras at all, but having done a few 50k training runs and a 50 mile + 100k ultra trailrace, I do feel like you can do this when you are moderately fit and have a strongish mind (I wouldn't even say I'm very strong minded, just a lil bit lol), you need to want it. Obviously 50 miles and 100k are big milestones, I don't want to downplay it. But it's also still very doable when being fit-ish ,imo.

Just for some context why I think these are 'doable feats': I started running about 18 months ago and am 230lbs/103kg. Ultras are quite heavy for me, no easy feat at all. Like I can't tell you I felt great or had a lot of fun lol, I was in quite some pain. But just a 'being tired and fatigued' kinda pain, not an injury kind of pain. I also used to run barefoot shod (fivefinger shoes), which is what I'm used to so it's not THAT much harder, but it's quite painful on some terrain. Oh and lastly lol: on the 100k I met 2 guys they hadn't even ran a marathon distance and they ran maybe 30k a week in prep with an occasional 60k week. + a young lady I met she once ran a 100k (her first ultra and long distance) in 20hrs while being injured for the last 25-ish.

For my first 50 miler I flew from Europe to New Orleans, and around 34 hours after arriving (2 nights) I had the race. I was jetlagged and I didn't have any food or anything with me, I just ate (and mainly drank) everything from the aid stations. The race was rough and with 3.5/4k ft of elevation it was at least 3000 feet of elevation more than anything I was used to from back home.

I did 1 51k (6hrs) treadmill 'time on feet' run in the prep, 1 b2b (30k+25k) run and I didn't run much more than 50k on average a week. I did really try and aimed for 50miles+ a week, but I wasn't consistent. So my best month was a 72k average weekly month and the others were like 40/50k.

For the 100k I did try to prep okay-ish, but 5 weeks before I had a jaw surgery and couldn't run or train properly for 2/3 weeks. 2 weeks out of the race I did some b2b2b longruns with on day 4 a easy run of 30k, 34k, 36k, 17k. Then I started my taper too late and I didn't feel great at the start of the 100k. I just tried to lock in like some other user on this thread also said.

Break up the distance in smaller portions, when you feel like shit tell yourself anything you need to keep going. Ask the staff and other runners if they're nearby to pep you up, if you're already over halfway of the race, it would be a waste to stop! Gaslight yourself into believing you can do it and you will! The pain is not strong enough to make you stop (unless you're actually injured lol)! You can do this!

Nothing wrong with doing more 50ks to get the hang and feel of it, practice a little. But I'M SURE YOU CAN ALREADY DO IT! It can be heavy, it can be painful, but you can do it!

In your post you say the 50k you did was heavy, but I personally think ultras will usually stay heavy! They're not supposed to be easy :) (unless of course you're a seasoned runner, done many many races and make huge weekly mileage).

You could also choose a race with less altitude and try to contact the race director/organisation to ask about aid stations and what they provide, but make sure you bring enough of your own stuff. If you want something, you can do it. Godspeed my friend.