I would put duathlons up there with the most painful exercises you can do. From the gun its balls out until the end although there is a pace eliminate involved. Ive done the sportsmans duathlon many times(actually I am the founder of the race) so i know how to race it. It is so simple but still the same guys make the same mistakes. Tip- run the first .5km easy!!. Because it starts on a slight downhill guys run the first 500m at a crazy pace. I knew I wouldnt have the legs to go with the fast guys so I had to bank on the bike leg to bring me into contention.

Killian Heary and Paul Carroll showed up..I was happy with that because I wanted a race to test myself. Two boys are handy and clean up wherever they go. Surprisingly I held them to 20 secs give or take on the first run and got a gap of a minute  until the second run. I wasnt entirely comfortable on the bike but i turned a good gear. I wasnt far off my fastest time on the bike which also surprised me. I think the ‘glow’ faster lid made the difference… sorry-bad joke!!  Fortunately for me the run leg is 3.1k as I blew completely after the first 1km of the second run. I held on just about with Paul nipping infront of killian. Good hit out that shows my power is there on the bike. The running is working progress but I have some speed.
more tips-
practice your 3km pace before you enter these races. You can get sucked into the fast pace but really everyone dies and it ruins your race. The same applies for the bike. Cruise out, dont panic.. shoes on ..tighten shoes.. Breath.. after 5 mins you should be ready to ride hard. The last run you just have to GO.. 
You need to practice running off the bike after doing some hard efforts.. A session Rob always gave me (that we have to revisit yet) is 1 hour turbo. 20 easy-20 moderate-10 hard-10 hard as you can go…. then run as fast as you can for 10 mins….thatll set you up nicely!