Expedition Adventure Racing
So you've been adventure racing for years and you've pretty much seen everything. You don't even bother with sprint races any more because who wants to get all geared up for only 6 hours worth of racing? You eat energy gel for breakfast and don't bother driving anywhere less than 100 miles away because it's just easier to hop on the bike or jog there. You've tried Ironman triathlons but feel like you're just getting warmed up when the race is all over. Expedition racing is for you!
There's actually a category of races between 24 hour races and expedition races which you might want to try first. Anything around 48 hours is considered a multi-day race. The final step before expedition races, these races have everything 24 hour races have in larger quantities. Sleep deprivation will also be a bigger factor especially as you get into your second night. At this length, even small miscalculations in nutrition, foot care, bike maintenance, etc. can multiply into large race-killing problems.
But the real glamour lies in the most ambitious form of adventure racing yet conceived - expedition races. Expedition adventure races usually last 3 or more days, with some taking up to 2 weeks to complete. Training for such a race is a bit beyond the scope of this website (considering that I have not even done an expedition race), but like many things worth doing, working up to an expedition race is an incremental process. You certainly wouldn't consider doing one without having done many shorter races.
These races are often laughably brutal, covering 400+ miles over vicious terrain. Each stage might in itself be more challenging than anything most people will ever do. Sleep deprivation will be a major factor as many top teams will choose to sleep only a couple hours a night (for 6 nights in a row, can you imagine!?) and hallucinations are not uncommon. Rope courses are often epic and physically demanding. Basically these are pretty much the most difficult athletic competitions known to man, though many lesser events try to make that claim.
Expedition races are also unique in that they occasionally get TV and media coverage and that there are professional adventure racing teams who do these type of races for prize money. Though there is not really much money in adventure racing to go around, there are a few hardy souls out there making a living off of prize money and sponsorships. In the early days of adventure racing, these races actually got quite a bit more media interest and had pretty nice prize purses. But there has been a migration in recent years toward shorter, more locally-based races rather than the massive expedition races. The reason they have fallen off in popularity may be partly due to the requirement of such a big time and money commitment to enter such a race and partly due to the fact that very few athletes in the world have the combination of fitness and skills necessary to even attempt one.