I agree with the other posters, don't ride so fast and you will last longer. Going distances is also just like any other kind of training. You are doing specific training for your triathalon sport, but you are not training for endurance riding. Some people seem to have a more natural endurance "gene". I do not. I have always had to make an effort to go long to feel good going long.

I can usually always ride 2 hours at a high pace, but over that, I get fried. Eat more on longer rides than you think you need to. second, train for long rides; so you are going to have to go on those weekend rides that are longer.

Last year, my goal was only to get my climbing and endurance training in cause I was doing the BIcycle tour of colorado and I wanted to do this without a lot of pain every day. So, I rode longer miles at a slower pace and did lots of hills. I basically trained for the specific thing that I was trying to do.

Hope this helps