Our expert says:
It is quite a bit tougher on the mountain bike than the road bike, yes. A lot depends on the bike, because the top of the line mountain bikes these days are about as light as some road bikes, and so you can then get away with it.
But generally, the extra weight of the mountain bike will be quite a hinderance. Can't put a number to that, it all depends, but I'd guess that it's a good 5 to 10 minutes per 30 km. On hilly routes, maybe even more.
So if you can get a road bike, I'd certainly advise it. If it's the iron man in January that you're talking about, then you have enough time, you just need to be methodical about how you practice. Once you've learned it, it's really easy and you'll feel just as comfortable on the road bike as the mountain bike, so you just have to commit and give it a go.
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