Yes, time based training is a good idea when you have something so technical or so much gain the route takes so much time to complete.  So many of my outings don't count as Ultra mileage, but a 20 mile outing that takes 6 hours could be Ultra time.

Not much help with a plan.  I have never been able to follow one with my schedule.  I think one can get by with less mileage if adding more intensity, specifically hills, but there is something about high mileage and the big long runs that build mental toughness.  That is key when going up to that 50 mile distance.  Miles 30 to 40 can be rough.  You start doing the math in your head, you start getting tired, you start getting into the heat of the day.

Don't stress about the mileage or the distance of the long run.  You have done a 50k you should have no problem with a little 24 miler.

I am concerned that you have too many races on the schedule.  While fine to use races as training runs for a longer distance, the tendency is to push the pace too early.  Also consider the recovery time.  I would suggest fewer races and more do it yourself Ultra distances.  Less stress, you can do your own thing, you can experiment with gear, etc.  Being out on your own without support builds mental toughness.

advice from a mid-pack intermediate here. i agree with Beryl on the plan. ive never been able to follow one either...but i love to run so that gets me training as much as i have time for. plus with a plan be careful not to try to fit yourself into something that doesnt fit you. i think people get hurt that way. if you know yourself, then arm yourself with some knowledge and make your own! i do recommend reading Relentless Forward Progress by Powell. there are some plans in there but they are deliberately kinda loose. IOW give yourself some latitude. figure out what kind of mileage leaves you feeling good enough to keep it up. the other thing i can say about 50M is that it is definitely a different beast for me and involves a ton of walking. i found the formula: 'multiply your marathon time x2 and add two hours' to be a good general predictor of time. this also gives you an idea of what things happen after 50k

Does that matter if I'm a walker with just a little bit of running thrown in?

I’ve run several ultras pretty much 60-70% then exploded and hiked it in.  Avg paces come out to that many could rather easily walk the whole way (15 min-16 min mi).  You could argue mostly walking with running makes the most sense for a majority of the field.  Though these were 100ks and a 100 mile same rule applies.  Plus, I would guess recovery is way better.  I think if you plan and execute it like that it will be a better time.  Goes withou saying if u plan to walk a lot in the race, I would walk a lot in training.  You could cut your time down significantly if your not already walking a lot, and it’s biomechanically different so different muscles/economy to train.

 

Walking is fine but at some point you are going to want to improve on your running and make running a bigger part of your run/walk ratio.  Mainly because of cutoffs and most trail races have them.  You don't want to always be fighting those cutoffs, too much stress.  The good news is that you don't have to overthink it.  The more time on your feet, the running will come naturally.  Don't be in too big of a hurry to register for that 50 mile distance.  I say get a few 50k distances in and some longer training runs.  Make that first 50 a good experience.

I'm moving in the opposite direction with more and more walking. Cutoffs have been a problem for me in some races. I have two DNFs because of missing cutoffs. I've also resigned myself to not being able to enter some races because I know I coudn't make the cutoffs. That includes several 50M races. On the other hand when multiple distances are offered, the cutoffs for the shorter ones tend to be very generous because they have to keep the course open for the longer distance. In some cases that has meant choosing a non-ultra distance like 21K or 25K, but that still gets you on some of the same trails and part of the event. I've yet to enter a fixed distance race longer than 50K as a consequence of all that. All my 50+ mile races have come in fixed time 24-hour events.

 

I'm moving in the opposite direction with more and more walking. Cutoffs have been a problem for me in some races. I have two DNFs because of missing cutoffs. I've also resigned myself to not being able to enter some races because I know I coudn't make the cutoffs. That includes several 50M races. On the other hand when multiple distances are offered, the cutoffs for the shorter ones tend to be very generous because they have to keep the course open for the longer distance. In some cases that has meant choosing a non-ultra distance like 21K or 25K, but that still gets you on some of the same trails and part of the event. I've yet to enter a fixed distance race longer than 50K as a consequence of all that. All my 50+ mile races have come in fixed time 24-hour events.

Good point.  Can’t miss the cutoff in a 24 hr race.  At Tunnel Hill the last 50 mile finisher was I think around 25 hours and in his 80s which is impressive.  Ultrasignup results