Is marathon pace useful in training?

running at marathon pace isn't going to help your 5k pr. you would probably benefit from adding another workout day - tempo run or another interval session. read up on 5k training plans

What would you say my tempo pace is?

Yes, I think there's a lot of value in adding something longer for 5k, whether it's mile repeats or tempo runs. Plus you're adding a second hard effort, which would probably help fitness whatever it is.

MP is fine as long as it's reasonably hard for you (don't use MP from a race you blew up in).

What speed work do you do?

If you have the time first take 6-8 weeks and develop you middle aerobic zones properly and strength with some progression runs, fartleks and hill workouts (all of these should more in the 70-80% effort range maybe a little 85%). I would do one more strength based session and one more aerobic development session a week (so 2 sessions a week).

Then progress to threshold repeats (8 x 800 at 85%, 7 x 1k at 85%) and start to run some broken tempos (2 miles at 85%, 1 at 80%, 1 at 85%) or In and Out miles ( 1 mile at 80%, 1 at 85%, 1 at 80%, 1 at 85%). From there add in speed work (800, 1k's, miles etc. all at 90% effort) along with a tempo workout every other week (start with 3 miles at 85% and ideally build up to 5-6 miles).

When you get closer to race time do some shorter stuff like 16 x 200, 12 x 400 at 90%). You can also do multi stimulus workouts like a 3 or 4 mile tempo followed by 8 x 200's for example....these are great race prep workouts in my opinion.

Do 8 x 100m strides at the end of ALL your easy runs

Easy days easy and the workout days and long runs at the correct effort....pace doesn't make you fit correct effort does!

You are doing the mistake as many to think mileage is the key for you to run a 17:30 5 k . And marathon pace is a none case in my training system . To run your sub 17:30 5 k you need one maxVO2 and one Lactate threshold interval session per week.

What paces are those 2 sessions?

What can you run for a mile? Can you run 5:38? Because that is race pace. Can you run around 5:10? Because that is the ballpark you need to be in for 17:30 5k.

If you're not seeing much difference with increased mileage, then you need to start working on speed. 200s (38-39s) & 400s (76-77s).

The maxVO2 interval at 5 k race pace and the LT interval at close to half marathon race pace. 🧙‍♂️🇸🇪

I don't think that steady runs at marathon pace are particularly useful for 5k training. It might have some benefit, but it's taking away from other more beneficial things you could be doing. I could see it being useful during base building if the overall intensity of the block is pretty low. I have my high school athletes do 2 workouts and a long run every 10 days during base training. That means they have 3 "easy" days in a row, but I assign the middle day as "easy/moderate" meaning they are free to pick up the pace if they feel good. That day could end up being close to MP, and I feel okay about giving them that bit of flexibility since the overall intensity of the block is fairly low.

Marathon pace (which I just refer to as "steady" since high school kids would probably slow down if I used the word marathon) is more useful as a "recovery" pace while doing faster than LT work intervals. Here are some workouts I assign my runners:

minutes Fartlek - 1 min around 5k pace/1 min steady

150 @ 1600 pace/650 steady

1 min @ 3200 pace/ 2 min steady

All of these workouts are meant to be lactate threshold type workouts with a few added benefits:

1) They work better for the average high schooler. Good luck getting a 9th grade first-year runner to run a 20-minute tempo.

2) They have greater neuromuscular benefits than holding a steady pace for a long time.

3) Athletes think they are more fun.

Of course, holding a moderately fast pace for a long time is an important skill, particularly for a 10k+ runner, but I think runners focused on shorter distances--particularly high schoolers--can benefit more from this kind of workout.

I'm still having a hard time understanding this.

My 5K pace is 5:50 and my regular "easy" pace is 8-8:30.

Wouldn't adding in some runs at 6:30-6:50 improve my endurance, because it's almost 2 minutes faster than my regular pace? When I look back on my training it doesn't seem to make much difference but I haven't tested it extensively, I've mainly just been doing intervals or easy.

Yes, adding Marathon paced running will improve your endurance by a lot. Because Marathon pace is slower and therefore not as taxing, you can do a lot of marathon paced work in a day. 40-50 min marathon paced tempos are great workouts, additionally, they help to clear lactate almost as well as half marathon paced tempo or reps at half marathon pace, and you're able to run more volume at this intensity.

Marathon pace IS tempo pace.