IMO "fitness" needs a qualifier, i.e. fitness [i]for[/i] something, especially when you consider the definition of fitness as "suitability". So a runner/cyclist/climber/strongman could all be very fit, but be rubbish at doing each others jobs.

Resting heart rate is probably as good as any simple test, but obviously that can be affected by a lot of things.

I'm a fitter endurance cyclist then when I was a fit rock climber so it's all different

This is why I accept that any measurement would apply only to me.

I mostly mountain bike (and this is element I would want to be best at). 1 1/2 to 2 hour rides 2 or 3 times a week.
Running - 10km 1 or 2 times a week.
Gym - 1 a week - light workout mostly doing core and upper body.

Not IME.

I can comfortably(ish) run a half marathon, something I wouldn't have been able to do 10 years ago. But a 90 minute competitive football match would kill me now, something I did twice a week 10 years ago!

To answer my own question I think I'll do a timed lap of local trail centre (Aston Court with 50 acre). Then once or so a month try and beat that time.

Resting heart rate is probably as good as any simple test, but obviously that can be affected by a lot of things.

Resting heart rate is *completely* useless at showing how fit you are.

I use an 8km local loop as a guide to how many shortbread fingers I can have with a cuppa. There's another hill near us that is a good gauge as well, if I can get up it on my SS (34/19) without my head exploding then I'm in ok shape. The 'do my jeans still fit me?' test is handy too.

I was once told it's how quickly your body returns to normal heartrate/breathing after you exert yourself

I think your question is not about "measuring" fitness as "defining" it. It's a very general term that can be applied in a range of ways. What do you want to be fit for?

I mostly mountain bike (and this is element I would want to be best at). 1 1/2 to 2 hour rides 2 or 3 times a week.

I'm happy with the amount and type of exercise I do. It would be nice to have a method of knowing when /if I start going backwards.

As noted, fitness is a relative term but a general indicator of one person's fitness over another's or indeed increase or decrease in your own fitness levels is to time how long it takes to get from 'your' maximal BPM to 'your' resting BPM rates.
I would suggest raising your heart rate by running hard and measuring your resting rate by laying down in a moderately temperature room and a comfortable position.
That's about as good an indicator as you'll get for a massively generalised indicator.

I would suggest raising your heart rate by running hard and measuring your resting rate by laying down in a moderately temperature room and a comfortable position.
That's about as good an indicator as you'll get for a massively generalised indicator.

Repeat the max to rest x 10, just for added vomit factor 🙂

Crossfit workout "Murph" is as good as you'll get IMHO.

1 mile Run
100 Pull-ups
200 Push-ups
300 Squats
1 mile Run

Add a weighted vest for extra "fun".

If you are interested in running or cycling performance, then the only metric that counts is Strava. By which I mean your times. Nothing else is of use.

I could argue that I'm 'fitter' than my mate because my threshold power is quite a bit higher. However the fact I weigh 25% more and he can ride for days is also significant....

1 mile Run
100 Pull-ups
200 Push-ups
300 Squats
1 mile Run

I'm pretty sure that using that metric >99.99% of the population would fail somewhere very, very early on in step 2.

If we're talking cycling specific fitness then FTP is probably the best metric (of a bad bunch). Especially if it's an FTP you can actually hold for an hour.

Edit: FTP / kg so that it's normalised.

Resting heart rate on its own is not a good indicator of fitness. Yes, many elite athletes will have low resting heart rates but you can also have a low heart rate for other reasons. Actually if you do have an unusually low resting heart rate its worth checking with your GP. I had a routine health check a few years back where they did an ECG, the nurse doing it expressed concern with my 50 something BPM heart and referred me to a GP, just as well she didn't check it in the morning as its usually in the mid forties and I am no elite athlete!

I also have a huge heart rate range 45-195 BPM, even though I am pushing 50 but again its no indicator of fitness, its just how I am built.

Some fitness watches will measure your heart rate variability. HRV is is the variation in time between successive heart beats. You would think the more the intervals between beats varied the less fit you would be but the opposite is true, the more variation you see in the time between beats the fitter you are supposed to be. Apparently its an indicator of the stress your heart is under. Does it work... no idea, google heart rate variation and make your own mind up.

If you want to know how your MTB fitness is, find a trail that's not very weather affected (not muddy, not exposed to the wind). Ride it as fast as possible. Repeat to assess fitness.

Other than that, it's all an approximation.

1 mile Run
100 Pull-ups
200 Push-ups
300 Squats
1 mile Run

I'm pretty sure that using that metric >99.99% of the population would fail somewhere very, very early on in step 2.

Not if they're doing crossfit 'pull ups'...

Have you looked on eBay for a Fitas****ometer?
Try to get one that is in calibration though.

FTP

Bleep test is a good one for gauging fitness if you play competitive team sports like hockey / football / rugby etc. If I'm over level 12 I'm happy. Nowhere near that at the moment though.

Not much use for cycling fitness though.....

As said above, recovery to resting rate after a high HR is a good guide. I had some heart tests done a while back and the Dr talked about how it was a good guide to cardio health. So that and how well I cope with a few local hills on the SS, either when fresh or after a few hours, a simple strength-HR test and a good constant if the trails are dry.
Another guide is my average speed or how I feel on a regular loop when sticking to a narrow HR band in Z2 eg base aerobic ability, but that's getting a bit 'VO2' etc and I think wind and freshness of leg makes it more variable.

I think strength and cardio can vary independently of each other, or if they are linked they rise and fall in longer-term levels at different rates. That's just based on impressions from my varying pace on the SS vs road bike over the years.

Strava. Same route. Are my times better or worse on the climbs and long flat bits? That's my barometer.

Can you sing your favourite tune while keeping a pace within 10% of your best flat out climb? 😉

From what I understand it's the recovery time from various heart rate zones back to resting. Also it's the speed at which your heart rate increases from resting to a particular heart rate zone when producing a certain amount of power or watts. Which is very easy to measure on a bike with a power meter.

But being fit for one activity may not mean you are fit for another. Being "cross fit", a mix of cardio and strength exercise, is probably the most effective. And easiest to measure.

Not if they're doing crossfit 'pull ups'...

Ha, I actually typed and then deleted something from the end of my post about ugly crossfit kipping pull-ups.