(Original post by D.O'T)
First point to be made is that you cannot just jump straight into a Triathlon. The normal start point for training is the run, it is normally the swing section. The swing section is were the runners make up for time lost over the swim and cycle sections, and the bulkier com-peter's lose their leads. You realistically should be able to run a marathon before becoming a triathlete, as it will give you a competitive advantage (its also needed if you decided you want to become an Iron Man). You say that you are a lifter, so I presume that you are unlikely to be a great runner (correct me if I am wrong). Therefore you will need to be a great swimmer and cyclist. These are what most people find tough, so you can build a good lead if your quick enough. I found the cycling was relatively easy, but the swimming is tough (even for a big b****** like me). Its not so bad if your swimming indoors, however the open-water swimming is physically demanding.
The diet usually consists of high carbohydrate intake (oats, rice etc.) to provide enough energy for a long period of time. The bulkier com-peters also have a higher intake of protein. However, the key, is the fluids. Hydration is a major factor to the performance on the day, however too much will make you sick.