What sales advise is correct. 10km is your ideal race distance stopping point. You could try 21km, but it’s not advisable to do long runs beyond 14-15km.
This guy did his first HM, running a time of 1.20. Longest run was 13km before. A lot of factors to consider is that he did this in Korea (better weather), was ex military regular and firefighter. He mostly runs short distance intervals, not many reps but very fast pace.
With this in mind, how to go about it?
Do short distance explosive intervals. 400s/200s, focus on speed. Long rest (2-3min), short reps (6-8 sets). Cut down on the rest over time, keep the pace the same. You run this at your 2.4km pace. Slowly scale up to 5km pace and do 800s. Similar 6-8 sets.
Don’t really need to go beyond that, since you will spend too much time running. This is the most efficient gains where your speed work will translate up to long distance and not losing muscle mass.