Very cool that your husband is going for a 24 hr solo.

I did my first one 2 years ago (hey pedal wrench, where do you race?) and became addicted.

To train for 24 hr races i work with a coach but can offer the following advice:
- Figure out your nutrition strategy. I love hammer nutrition -- and their endurance group (free email group you can join) has really helped me in this aspect. Aside from fitness and endurance this will have the biggest impact on race results.
- do longer (4 - 8 hr) race sims. This will ensure you test your endurance and your nutrition plan. Eat like you will in the race (can't stress that one enough - a huge part of these races is (for me) keeping your stomach happy). If you have races available to you for these 'race sims' - even better.
- do some longer rides on the road (endurance) but most on the trails - you need to get used to the beating that your body will take over all the logs, rocks, roots, and etc. no amount of fitness can compensate if your upper body can't handle it. I like to train on the hardest course around - the one that offers loads of hard climbs, a mix of single-track and double-track, and that beats up my body (technical skills required). This prepares me well for the harder 24hr solos. For a first-timer i'd try to pick something that simulates the race you are aiming at ... for me my A race (24solo) is worlds so I pick the hardest routes around to train on.
- not everyone does this but I like to do a bit of running up really steep stuff for cross training. in 24hr events you often get off your bike ... if you are fit and can run up the hills or have no problem with walking fast you can pass competitors.

my 0.02