I'm actually jealous. I would love to commute off-road on my MTB! Although that probably turns messy on rainy days. Take it gradually, 18 miles one-way is pretty long commute, and off road makes it that much harder. Rest, eat well and you will be fine. Don't feel bad when sometimes you need to take a day off from riding, you don't want to burn out and make this a chore. It should be fun.