There is a article in this months mag with a 3 month plan. I'm in a similar situation, currently recovering from a ankle op, but have entered twentyfour12 in July.
Maybe try and fit in a couple of smaller enduro events before.

having done a few of these in the past, i've found that offroad mileage doesnt help that much. your best bet is to get the mileage in on the road, dont get too concerned with time-what you need is distance in your legs, i'm talking 100+ miles at a time. long steady rides 😀

Little and often is the key I found, rides that is, not necessarily "training". Sounds like you're doing the best you can, and if you keep it up you'll be just fine. I did make an effort to get 3-4 >50mile MTB rides in in the preceding months. I also got used to eating regularly when on the bike.

You'll need a damn stubborn head and the determination that, when it all stops being any fun for a moment at 16hrs (which it will at some point) and you feel like you can't possibly carry on and possibly you feel a bit sick too, that you're just going to keep moving, slow down and eat your way out of it.

Someone in the pits to put you back on the bike and just make you keep going round, no matter how hard you protest, is a very good thing, they must be prepared to be be sworn at in the early hours of the morning and can expect to be thanked profusely during sunrise and at the finish line!

I absolutely disagree with the advice that off road mileage doesn't help you that mush, you'll be spending a long time in a mtb riding position, with your body taking a battering from the terrain, you'll need to get used to that. Sure, get the hrs in on the road to to mix it up, but long steady rides can be done on a geared mtb too – that's what gears are for.

FWIW I entered my first ever 24 solo last year (SITS) following that advice that was passed to me by several experienced and successful endurance racers – and I won!

everything rob says is the truth !

i did no mega mega long rides – longest being 90kms (bealach na ba) in november for the strathpuffer !

just get out on your bike often – i was riding every day nearly – but only 20-40k a day.

Completely agree with Rob (you'd be stupid to disagree with someone who has won one of these things!) however, I recommended running as it's easy to fit in.

I live in a flat so getting the bike dirty means I have to hose it down etc which is a pain in th ass of a night and I work a lot of hours and don't own a road bike. If I get home at 7 I can be running be 7:10. If I do a 10k I'll be back and stretched out by 8:15, showered and on the couch/cooking for half 8.

I know it's not ideal but it's better than doing nothing!

If you're reasonably fit, you can do it, so stop worrying about that part.

Training serves to improve your final place.

If you aren't worried about placing, ride "slow" and don't stop, but keep eating and drinking. That will guarantee you will do more laps than some younger, fitter, faster riders. It's about mental attitude, and once you have one 24hr successfully under your belt you'll know where you can improve for next time.

(What I mean by slow is avoid any excesses of enthusiasm on hills and be careful in places where you can crash – there's 24 hours to get through)

A properly serviced bike is important too. It amazes me how many people have unnecessary mechanicals during a 24 hour – it's almost a whole season's racing in one hit. A mech problem can cost you an hour, so it's worth avoiding.