In my city, there are so many pockets of mining land, now nature reserve, you can actually cross the whole city on a mountain bike and stay mostly off-road. And I've heard of a few hardy souls, who regularly commute to work on their mountain bikes, clawing time in their work day for fun and fitness. In my usual way, I will go too far now, and suggest we all sell our cars and treat shopping errands like hunting trips: "here comes daddy, back from the shops, with a skinned and gutted elk slung over his shoulder." We could use these networks for all of our trips: "Sorry teacher, my kids will need a late note, and we did encounter some mud this morning, getting to school." "Your Majesty, I would like to introduce you to Dr. Behooving. He is always covered in mud, as a political statement."
My real point is somewhat more tame, and it is that any trip made by bike through a nature reserve, is doing less damage to the environment than a trip made by car, on a sealed road. Roads are an enormous blight on the landscape, serving two tonne noisy machines, chewing up more energy than will ever be produced via renewable sources. These days, mountain bikes trails are designed to prevent erosion. They have zero impact on the broader environment, plus a positive impact on physical and psychological health. Let's view mountain bike trails as transport infrastructure, albeit for a limited number of patrons, and provide end of trip facilities suitable for hosing down, and safely storing, these bikes that—beneath the mud—are the most technologically sophisticated devices anyone could choose to commute on.
Further reading: Last year, I took this idea about mountain biking to an urban design conference; see pg 82-88 of the proceedings. I've also written before about mountain biking appealing to that old devil Thanatos, while upright cycling appeals to Eros, and road riding is Narcissistic. Here's a blog post on those Freudian readings of what motivates us to ride in various styles.