Body Composition and Weight Training for Ironman Athletes

 You can certainly weigh heavier with doing strength training in your program, however it should be from lean muscle mass. As an endurance athlete there is a balance. Obviously the race is LONG so carrying extra weight is hard work.
If you are a runner, the smaller you are, the better. Swimmers carry a little mass, and they need that in order to swim well. I've seen many pro triathletes ruin their season getting too small, too lean and killing their ability to swim at all. Cycling is a balance, you want to be lean but too small and you lose power.
So, triathlon is unique and in particular Ironman triathlon is unique.
If you look at the top men over years who race well over the Ironman distance in their off season, or in their natural build, they are a little bigger guys. Chris McCormack, Peter Reid, Dave Scott, Ben Hoffman. All these guys, if you go back and look at their really old pictures from when they started, they are all pretty "big" guys, and get a bit bigger in the off season.
An ITU racer is typically a lot smaller. However, even they have to be careful to not kill their swim getting too lean for the fast run.The "bigger" guys of Ironman racing, over a long period of time, morph into a smaller, leaner frame. However, naturally they still would get "bigger" soon as they went back to normal life.
There is an element of needing to be a bit stronger for these super long races like Ironmans. So, what does this mean for us normal folk. We need to be strong, yes, but we also need to be a size that doesn't kill our run to lug across a marathon, especially if it is hot out.
In most cases, if you eat the right foods for performance, and it is timed well with your training, your body will evolve into the right body shape and weight. Of course as we age that gets a little trickier. Maintaining lean muscle mass as our hormones change can be a challenge.
If you are eating truly very well, following a good training program, and your body composition is still not evolving the right way, you can always hire a nutritionist to give some feedback and guidelines. Most people have daily habits that kill them more than they are aware of. Keeping a nutrition log can help with this. I do think weight training in the off season for older athletes is really important to combat losing lean muscle mass and overall health. Getting enough protein and recovery into your day is important too. The other factor is staying hydrated. Being hydrated lays a big factor day to day on your recovery and your body composition.Start by keeping a food log for a week and see if you notice daily habits you can change to improve your nutrition. Start there and we can build from that point forward.