Is a 2.5 meter RCA phono run okay for an MC cartridge with an output impedance of 24 ohms?

No

Well, honestly the biggest issue is noise, so you could try it, but I don't know if you'll like it. If you are using a separate phono preamp, put it as close to the turntable as possible. Otherwise, it's kind of a crap shoot.

I added a 3rd tonearm, backwards on left side, thus needed a longer cable, had this company make me an 8 foot one, sounds great.

https://www.ebay.com/itm/Custom-Length-Audiophile-Turntable-Phono-RCA-Cables-w-Ground-Wire-Quad-Brai...

that's rca to rca, he made me a custom din to rca, Jesse is a very helpful person

When dealing with phono cables, shorter is always best. the signal is so tiny that physical parameters and their resulting complex dynamic LCR, under load, will tend to swamp the signal and distort or color the sound. Far more than would happen with a line level signal and the same cable.

Imagine a train wreck trying to deliver a chipmunk. Chipmunk:tiny..., train wreck:huge.

However, these are perceptive/perception things, in the end.. as we’re not going to sit there and measure measure measure it all and then analyze it with a coven of engineers and reams of test gear..... and then add in what we hear.

it’s all too much, overall. We’ll just listen and decide.

Essentially chakster’s answer, is my answer: No.

Move the phono preamp or reorganize the room. Far better results that way.

The only way to know it truly, via listening...is in single cause analysis, via the use of two set-ups that are identical.. with the single change of two different lengths of the same cable. and then try each, and evaluate via ear.

Repeating this test in various guises and situations over many decades, says that the short phono level signal cable is best for audio quality.

So, it’s a no, and not that ’opinions will vary’, or that ’it works for me’.

In the final analysis shorter is better. In all properly done experimentation (single cause) on this subject..the final analysis is..shorter is better.

Do what you want and use whatever lengths you want but understand that a long phono cable IS a compromise, and there’s just no way, in the world of -physics, cable geometry, complex impedance, and signal level and analysis - to get way from that.

One can try the given longer cable and feel it sounds just fine and is very much acceptable. A perfectly fine evaluation method -by any standard. But it is still a compromise.... 

Additionally this sounds like a very low output MC cartridge and noise will be an issue, as will clean dynamic range that is consequently compromised by dynamic cable interactions in the LCR and molecular noise areas of consideration...