50 mins is can lah..... just need more focus on threshold work. You'd surely have the time to spare aside from work/family since you are more into 21k
Actually instead of 2-3 china shoes, invest on a lactate meter. $300 full set can get from TB already.
I feel it is probably what got me the most and fastest improvements i have seen for the 10k last time when i was still playing this distance, even for 3k/5k i believe the importance is quite crucial though more indirectly. Coz you are testing in the frequency of every 7-14 days, you can see the progress and most importantly it is not guesswork, it is a figure that you see on the meter. People can take years to get from 55 mins to 40 mins or they can take many times faster in just months. Of coz, this is just a tool that you use in your structured training, but it is a pretty important tool......just dont kena analysis paralysis.
Or put it this way, easy days too hard and hard days too easy (esp for threshold) - wont happen.
And once you have the correct info/experience and things dialed in for training, it's not that difficult to hold onto these timings once you have achieved them as PBs, just a matter of consistency rinse and repeat the routine.
Or put in another way, you can run 4-5:00 paces "easily", wear the elite shoes also more shiok.
Actually my most recent past 1 year plus of training in calisthenics, "training too easy" is quite an easy trap to fall into. Once past the beginner gains stage, you really can stagnate easily if you do not have a hard data reference point (ie reps and weights) and not improve at all. Of coz also got many other factors as well, most impt of which is recovery.