Used to run a lot in my late teens early 20s bit only up to 15 – 20 miles, now in my 40s and i have not run for years but do enjoy racing 12 hr solos. Out of interest anyone participate in both disciplines and which do you consider the harder to do?

Hmmmm tough one. If I take one of each of those that I've done I'd have to say the 12 hour.
Definately felt tougher doing a 12 non stop i.e not coming off the bike except for a wee. That's comparing it to the Snowdon Marathon, and baring in mind I'm a cyclist first.

having done one London marathon and coming up to my 7th solo sleepless in the saddle, the second to be fixed wheel, i'd have to say the marathon is the harder one, though i didnt do any running before hand, so you would expect it to be harder 🙂

I've done London marathon once (this year) & solo 24 a few times & solo is harder mentally, but my muscles were more tired running & it took longer for them to recover from running.

Running will always be harder for me – & most people I would guess

But 12 hours cycling Vs 4 hours running skews it the other way for that comparison IME

Not telling – I think you need to do it to find out 🙂
I've done both and they're just different. I think it depends more on you, your fitness and your preparation. Also depends if you want to get round or race.

12 hour solo is harder. training is "bulkier" ie training for a marathon takes up way less time than training for a long MTB race. 12 hour is way harder mentally. Its not like you will get tired running after 4 hours and fall off or hit a tree.

If you have done a half marathon then a full one is about 3x the effort, not twice as hard ie the second half of a full marathon is twice as hard as a half marathon (will make sense when you do one, you should do one)