How long does it REALLY take to start losing fitness after not working out?

It takes 3 days.

I monitor my HR every single run and have done for 4 years.

If I take more than 3 days off, it’ll have a direct impact on my HR for a given pace. I took 2 weeks off recently. It set me back 4-5 months in terms of fitness.

I’ve also come to realise that even a really slow 3 mile recovery run helps overall fitness rather than taking a crosstraining day.

Everyone is, of course, different.

If you are sedentary besides your running, three days. If you walk three to five miles a day as part of a normal routine such as a college student on a big campus, five or six days.

Only one answer... wrote:

It takes 3 days.

I monitor my HR every single run and have done for 4 years.

If I take more than 3 days off, it’ll have a direct impact on my HR for a given pace. I took 2 weeks off recently. It set me back 4-5 months in terms of fitness.

I’ve also come to realise that even a really slow 3 mile recovery run helps overall fitness rather than taking a crosstraining day.

Everyone is, of course, different.

Same for me. Took 8 days off after season, and needed 2 months to get back to where I was before. Biggest mistake ever. That was at the end of my first year of running.

People who have been running for 5+ years, and 15,000+ lifetime miles won't lose fitness as fast. But someone who is new to the sport and just started building a few mitochondria can lose them very quickly.

I think the general opinion is that a well-trained person only starts losing fitness after 10-14 days.

Taking one week off may result in supercompensation due to the tapering effect.