What Munqe-chick said. The Helly Hansen ones are great fun, are NOT easy but are VERY do-able. I've done a few of them with mates and we've always had a good time – great atmosphere and not really that competitive an environment (there are a few mental teams that do them that always win!!).
sleepmonsters is a the adventure racing website that most AVers in the country use. Might be worth looking at a short off-road duathlon to try first like maybe 5k 20k 5k to get the feeling. Don'tworry about the kayaking – most of the kayaks used in the organised races are a bit crap and the effort required to kayak really well is silly. You never really lose or gain much time in them anyway.