Stuart - hubby and I do distance base in both running and swimming throughout the winter, with mountain biking at weekends, then start triathlon build-up around New Year. We usually do a couple of duathlons in spring, and here in Germany the tri season opens around mid-May. If training for an Ironman, we do most of the distance stuff at weekends (if no races) and speed sessions during the week. And don't forget strength training - we use lightish weights and lots of repetitions. Doing an Ironman schedule, you can get strong enough to carry you through a decent marathon, but if you really want to post a PB, I don't think you can do that and work on tri as well. Good luck - you'll find the more you do the more you can!