Anthony - I have been doing various multi-sport races for the last 12 years or so from sprint to Ironman to 2-day races like Sea2Summit. I have found a great return for my CrossFit training investment for races up to Olympic distance but for half Ironman races (Wildflower, Pacific Crest) I have found that you need to put in your long miles to perform well. There is a certainly a cross-over effect in your endurance capacity from high intensity work, but don't expect to do lung searing, high intensity work that lasts for 10 to 30 minutes and expect to perform optimally in a 2 day long race...or an Ironman...or a half Ironman for that matter.
As to workout construction I have had good luck with a MWF CF workout (sometimes twice in a day with two different workout groups) swimming on Tuesday and Thursday morning at 0500 and then biking and/or running in the afternoon, a long run, bike or paddle on Saturday morning and a rest day on Sunday. Every 4th week I take a BOW or back off week and half the training load and then hammer again the following Monday.
No injuries, never sick and I feel great.