Z5 workout?

Stop with the stupid zone training.

I'd keep it simple, and start with an interval workout on that flat piece of pavement.

Something like 8 x 90 seconds at 3-5k race effort, with a 60 second jog recovery.

That's a 20 minute workout in total.

Your HR will be getting fairly close to 190 by the end of the last interval - but don't worry if it doesn't. It's quite difficult to reach max HR in a workout, and it's not even desirable. I'd recommend saving all out efforts for races.

You can gradually increase the total time of the workout each week, but don't go overboard. Perhaps increase by 1-2 minutes, so that the next workout looks something like 7 x 2 minutes at 3-5k effort, with a 75 second jog recovery. Then perhaps 5 x 3 minutes at 3-5k effort, with a 2 minute jog recovery, etc.

I wouldn't do more than 1 of these workouts a week, and I probably would only do them for a 10 week block at most.

I also wouldn't go any longer than 5 minutes for a rep (and this would certainly be at the slower end of the speed spectrum; 5k pace, as opposed to 3k), and would recommend keeping a 3:2 work to rest ratio.

Make sure you take a few easy days before and after these workouts, as they can be quite taxing, and come with a fairly high risk of injury (if not done properly).

Good luck.

You seem like a coach. Why only a 10 week block? What do I do after the 10 week block?

You tend to get diminishing returns after a certain period of time, and your body will need a change of stimulus in order to improve. There's also the injury risk, which isn't worth it, if the performance gains aren't there to justify the risk.

I'd recommend just switching back to doing more threshold workouts for the same period (10 weeks), and you can mix in some shorter hill workouts (10 x 200m with a jog back recovery, for example.) and the occasional steady run (slightly faster than your usual easy run pace). You can also try to gradually increase your mileage during this time.

Remember there are 3 major parameters for distance running performance: Vo2 Max, Lactate Threshold and Running Economy.

Zone 5 will improve Vo2 Max, but you also want to have periods of the year when you prioritise working on the other 2. This is how you ratchet up your performance, so that you continually improve over long periods of time.

As a new runner, what is going to give me the fastest improvement in times for 5k? When I started in 2008, I was seeing huge jumps in time from racing every run. I quit running for 16 years. Now I am mid 30s, racing every run isn’t giving me big improvements, but neither is easy running. It must change when you get older?

There's no magic bullet, but consistency and increased volume are the most reliable ways to improve.

Definitely don't race every run. Besides the high likelihood of getting injured very quickly, you're placing too much of a demand on your anaerobic system, rather than developing your aerobic system. The 5k is 95% aerobic, so that's what most of your training should be targeting.

The 80/20 rule is a good guide. That is, 80% of your running should be easy (purely aerobic), and the other 20% can be hard (relying on the anaerobic system).

That 20% can be made up of vo2 max workouts like the one's I recommended earlier, or they can be threshold workouts, etc. These workouts require lots of recovery time though, so you can't do them every day.

The other 80% should be easy jogging. A pace that you can "hold a conversation at" is a good rule, especially for new runners. If that's 11:00/min mile pace or slower, then that's absolutely fine. This way, you'll be working on your aerobic system without the stress and fatigue of hard anaerobic running (and the long recovery time that comes with it). Eventually, you'll be able to do easy running every day (sometimes twice a day), but you need to gradually build to this - or else you'll likely get injured.

Z5 means race