If all else fails, hammer the workouts?

Train don't strain.

Leave that threshold and lactate stuff to the pros. Run easy for 2 hours a day consistently 6-7 days a week (obviously youll have to build up to this gradually) and work in some strides, intervals and the occasional 5/10k race. You'll start seeing faster times.

I assume you're the same guy who's made threads about tempos at 6:30 pace. Yes, you need faster workouts. You can't race a 5k if all your workouts are slower than 5k pace. And yes, some workouts need to be really hard, don't listen to the hobbyjoggers telling you to sLoW dOwN.

If you've been doing the same sessions for months and haven't made any breakthroughs then it certainly seems worth changing the stimulus. Why not sub in one slightly harder session every other week and see how your body responds? Personally at only 5hrs per week I think threshold only probably isn't the best bet and you can afford to push a bit harder one session a week, you've got the recovery windows. Obviously in an ideal world where time was no object you would ideally go down the route one of the other posters mentioned and build up to higher volume first. But with 5hrs per week you're probably better changing the stimuluses.

yo, for me it was the same, trained for 3k and 5k
tried high volume and slow only
tried sub t only on 5-6h per week
raced slower than on 3.5h/week and i mean almost a whole minute slower over 5k... racepace felt awful and needed like 5 races at least to get somewhat in shape

some people like us thrive on hard vo2 or anaerobic reps. I need those year round to improve and stay fit. Also gives you most bang for your buck

my advice, stay off slower stuff like slower than 1h racepace! go polarized, jog around a few times a week and if you feel fresh hammer some hard workouts or even race/tt every week. This works for since middle school and im already low 15min 5k.....

dont listen to people who promote long and slow or sub t only. they usually need almost double the volume to run good times and need longer to get in shape

In what world is 5-6h per week high volume?

Also, never trust someone who doesn't use capital letters.

You’re probably doing your workouts too slow and other days too fast. Hammering workouts works if you’re disciplined to take your easy days easy and be serious about recovery.

Do you have any more time available to run? If you’ve been doing sub t for six months and you’ve built up to only 10 miles at pace weekly, that’s just not very much. My typical sub t workouts are at least 6 miles at pace, 3x a week. And if I want to, I can decrease the length of the individual intervals down to 400m or whatever and spend more time targeting paces at or close to race pace without having to hammer / get too lacticky / induce too much acidosis.

Do you have any more time available to run? If you’ve been doing sub t for six months and you’ve built up to only 10 miles at pace weekly, that’s just not very much. My typical sub t workouts are at least 6 miles at pace, 3x a week. And if I want to, I can decrease the length of the individual intervals down to 400m or whatever and spend more time targeting paces at or close to race pace without having to hammer / get too lacticky / induce too much acidosis.

My bet is that you're doing the workouts too slowly. Drop the paces significantly without getting below 5k pace. Do regular strides and have a day where you work the speed with hill sprints or 150s or 200s.

Volume is king, at least for endurance sports.

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Tempo runs alone aren't gonna make you race ready. Have you ever heard about race pace running?

It's hard to say to just scrap the training or not. Is it a traditional Norwegian singles looking week with 3 easy runs 3 sub-T workouts and a long run? 10 miles is a decent amount of volume. That seems in line with 20-25% of 40ish mpw, which I'm guessing is what around 5 hours/week gets you? At the end of the day you're not tapped out in terms of workout & overall volume, not even close really. A lot of 5k runners on that kind of volume do lack some strength so it's hard to just say it's not working. Maybe you have improved your threshold. Maybe you'd run a better 10k-half right now. I like the Norwegian method but even runners using it will sharpen at different points in the year. You might have a better base now to bring back some faster workouts & get to a start line with more fitness. 6 months isn't a ton of time & I'm not seeing anything to suggest that anything other than the 5k isn't improving.

3/10 obvious bait, but got a bunch of comments.

3 easy runs + 1 long run = 4 hours 20 minutes, leaving 40 minutes for 10 miles at sub-T? Doubt.

There is no one size fits all training program. volume can really harm some individuals. it’s also good to change the stimulus up every once in a while as your body adapts to a load. It’s best to find what works best for you.

If you're talking in terms of the Norwegian method, then yes, you need to have a faster element added. It's what Bakken calls an X element.

The whole point of threshold training is to enable you to do a good volume of quality while maintaining high global volume without getting hurt or overtraining. You can develop the aerobic system without limits that way. If you’re only doing 5 hours a week of total training, the global volume is low and so the total training load is low. And so you’ll probably need to ramp up intensity a bit to get the most bang for your buck.

70s style - insane mileage and some blazing speed work (but not much)

90s style - low(ish) mileage but all our intervals were basically at race pace. I was literally scared of the workouts our coach had us doing.

2020 style - high mileage and lots of threshold (but at a level that allows you to do tons of work)

Your style - low mileage and low intensity = not much improvement.

Like Ruxton is saying, the point of threshold-focused training is that you can do a lot of it without breaking down. If you aren't doing much volume, you should definitely train harder.

Not quite the same, but I cross country ski with people who insist on training like the elites do. Every easy ski needs to be strict zone 1. The problem is getting to 10+ hours per week is pretty hard for most people with full-time jobs. Many miss a couple days and end up at 7 or 8 hours for the week. It's more difficult than running the same amount since you typically need to drive somewhere. 10 hours is still pretty low volume for nordic skiers--you won't find many who ski under 15 hours per week. Results of the low volume/low intensity approach are pretty mediocre. The best high-level rec skiers I know aren't afraid to get the heart rate up on climbs and spend at least a little time skiing somewhat fast most days.

In other words, look at the whole picture of the training. Trying to take specific components without looking at how they fit into the big picture rarely works well.