Hey there, I feel bad that no-one has answered your question!! I'm an absolute slogger, and will never ever win any race I enter - just doing it for the fun of it, so I feel that the opinions I give you are good for anyone new to off road.
Road running is pretty tough on the body, especially when you're a bit weightier than you should be. I know that after a 21km or longer road run, my quads are hammered from the constant pounding, and Archillies are also in need of a rest. Off road has you moving all over the place, so youuse different muscle groups constantly - you might feel them the next day, but the load/impact has been spread. The only real problem is that you run a slightly higer risk of twisting an ankle or similar. But then, on the road, you have a much higher risk of being hit by a car than off-road!!
Also, you're constantly concentrating on the next few steps off road - keeps you sharp and thinking. The ground is always undulating, and a rock/hole can appear out of nowhere. Depending on how hard-core off road you're going, you might end up with some real rock-hoping or climbing tactical stuff, which is cool.
Tips - use the road to train for off-road; not vice-versa. Go with a couple of other people; lonely out there!! Take a waterproof map of the tracks, and study it before you go - sadly, this is something I've learned the hard way! Take a Hydration Pack for water with at least 1.5l capacity. Take some muesli bars for sustinance. Tell someone where you're going if it's really in the middle of nowhere. Take a cell-phone in a plastic bag. Be prepared to get muddy and wet. Get some good off road shoes - inova-8's or brooks or similar. Lastly, just get out there and do it; leaves road running for dead!! Its a big beautiful wilderness out there, and this is a great way to see it!
