Until 6 years ago I was a mountain biker and I can assure you it is hard. Providing you put enough into the offroad and find the hills- Road riding is tame.

Imagine your worst hill- on a bike that is around 10 lbs heavier- the terrain being a bit rutted and a lot uneven- and with WIDE knobbly tyres. OK- the tyres are necessary to give you the grip across the rocks and scree so discount them. But Mountain bike Hills are harder.

But have to admit that in my area we do not have long hills. Longest on the road is about 1 mile at 10%--shorter ones are steeper. Offroad and the Easy climb to the top of the hills is 1 mile long at an average of 9%. That discounts where it gets to 18% at the end and the trail being rutted from the rain and piles of scree 2 to 4" deep that sap the energy out of your legs when dry and test your balancing ability when wet.

But when I changed over to road- I did not find it easy. Hills are hills and they need some adjustment to do. However I did go to the Alpes and climbed Ventoux. My training for that consisted of hill repeats up the toughest hill in our area. It did not prepare me for a continual 13 mile climb at an average of 7 1/2% with some 10 to 12% thrown in. I found it hard despite all the training I did but without that hill training-I would not have done it.

So on your bike- Find the toughest hill in your area and ride it- again and again and again.