Here's my advice -- not necessarily specific to adventure racing but rather to any event with a life partner.

I suggest you and DH discuss your plan for the event in advance. Be very honest with each other. What will you do if one of you can't keep up. What if one of you gets sick. Or injured. Or cranky.

Beyond that, I would also try to visualize the entire day with your DH. Try to plan for as many options as you can.

An event like this could be very high-stress and a real strain on a relationship. If you both understand that, address it in advance, and have a "safe" word (ie something to say when you just need to remind each other that you love each other), you'll have a more successful event!