I do one, maybe two duathlons a year. A chance to ride the TT bike. The recent local yokel one I did was about 1200ft elevation in 25mi of the ride and I did 24.5mph, including braking down a hill for a while for slow car traffic (just bad course design there).
The two 5k runs? 22:05 and 22:35 on a rolling course. Some decent little hills included.
That was good enough for the win by something over 2min. The real fast folks at this are doing the bike at about the same but the runs more in the 18min per 5k pace. Meaning in a more elite event instead of local I would have gotten smoked by about 6min.
All that to say......you have to do tri/du/time trial stuff as personal goals and personal bests.
You can snoop the finish times then find folks on Strava and see what their weekly hours are by activity type. Then set personal goals for training/finishing from there. With a mind to what age the folks are also.