Haven't done one myself yet, but I trained with multiple IM athletes for the past 3 years, and did most of their long runs and rides with them. Typical Sat workouts were 80-100 mile bike ride with 5000-8000 feet of climbing, or a 15-22 mile run with 2000-3000 feet of climbing, and a 2-4 mile swim at some point during the weekend.

A surprisingly large number of mortals in SoCal do IM. And a lot of them are very, very slow. Most folks also agree that the pain of doing an ironman is not as significant as compared to going all out for a standalone marathon.

It's a long race, requires a lot of training, but the 17 hour cutoff is very, very generous. You can walk the entire marathon segment at a leisurely pace, and coast on all the flats and downhills on the bike leg, and still make it so long as you don't melt down and stop.

Many of the multi-IM folks here would be unable to even hold 21mph solo for more than 5 minutes on the road. That's not really their fault though, as when you're training for a 15+ hr race, your ability to keep going matters much, much more than your max power. I also trained with competitive road cyclists on alternate weekends, and while they did shorter distance rides, they were MUCH faster than the triathletes, with plenty of sprints and super high intensity climbs. Training with both groups was excellent for overall cycling, I felt.

BeginnerTriathlete.com is by far the best resource for new folks aiming to do an IM, BTW.