How long before you start losing speed?

You need to be more specific. What distance and how fast?

Sorry. So my 5km PR is 18:41, 10km is 38:52. I'm usually on about 40 miles a week. I often find myself thinking if I've gone a couple of weeks and haven't done a workout or faster run that gets me running at that 5km or even 10km pace for at least a few kms, that I'll lose that speed or ability and will set myself back.

In many ways, the more pertinent question is, how quickly can it be regained. In my experience, very quickly.

ParanoidRunner? wrote:

Sorry. So my 5km PR is 18:41, 10km is 38:52. I'm usually on about 40 miles a week. I often find myself thinking if I've gone a couple of weeks and haven't done a workout or faster run that gets me running at that 5km or even 10km pace for at least a few kms, that I'll lose that speed or ability and will set myself back.

That's not speed that's stamina you are talking about.

True sprint speed (your 40m dash time) doesn't really go away quickly. It's mostly about talent to begin with. You can fine tune what you have, but you can't train yourself to be a Bolt.

The ability to run a strong 3k, 5k or 10k needs excellent stamina, which takes a long time to develop and also declines somewhat quickly after extended breaks or periods without any faster running.

There is a strong correlation between top speed and mile to 5k speed for a distance runner. If a runner can't run a sub 15s 100m, they cannot break a 4min mile and would likely top out in the 4:30 neighborhood with good training. That same 4:30 means they are unlikely to break 15 in a 5k. There are exceptions, but generally top end speed is a limiting factor all the way up to the marathon.

Do strides if you are not doing tempo or interval work. It is a low energy cost and keeps your top end speed pretty sharp. Then when you transition back to tempo and intervals, the speed endurance comes back faster. As you age, top end speed declines so strides are even more important as you get older.

ParanoidRunner? wrote:

Sorry. So my 5km PR is 18:41, 10km is 38:52. I'm usually on about 40 miles a week. I often find myself thinking if I've gone a couple of weeks and haven't done a workout or faster run that gets me running at that 5km or even 10km pace for at least a few kms, that I'll lose that speed or ability and will set myself back.

Why are you worried about losing speed when you have none in the first place? Do some strides and accelerations and plyos sheesh.

ParanoidRunner? wrote:

How long can you go without putting in faster workouts before you start to lose speed? I'm always paranoid that if I go a couple of weeks without putting in a tempo run, or a faster workout, that'll lose my speed and not be able to get it back.

Yes you will lose speed if you do nothing fast for a couple weeks.

A tempo run doesn't help "speed."

Thanks for sharing your experience! I'm pretty new to running, so this info is useful for me.

StrongCorrelation wrote:

If a runner can't run a sub 15s 100m, they cannot break a 4min mile and would likely top out in the 4:30 neighborhood with good training.

An understatement for sure. I've never met a 4:30 miler who couldn't get well under 15, including from a standing start.

a 4 minute miler should be capable of a 1:50 800 at worst, and a 50 400, at worst a 13 flat out of blocks for 100. Slightly slower than Nick Symmonds.