I did the Human Race winter duathlons last winter. They were damn hard – a hilly, off-road run of two laps of a course of about 8 to 10km, an 18 to 20km mountain bike ride and another run of half distance, 4 to 5km. The distances varied according to the terrain but I was racing for about 2 and a half hours on average.
Their ride courses aren’t like MTB races – they’re wider and a lot less technical on average. If you’re a half-decent runner and rider, you’ll be down the field after the run, recover a lot of places on the ride and lose a few to better runners again on the last run. A lot of the riders don’t use momentum and look for alternative lines so you can make a lot of places by just being efficient.
I’d encourage you to try it. Just remember to pace yourself carefully and keep gels and fluids going in throughout the race.