I recently went to my LBS to purchase my first bike. I haven't ridden since I was a kid, but wanted a way to get in shape this summer. I live only 3 miles from my office so I figured it would be a piece of cake to commute a few times per week. I eventually settled on a Trek 7.1FX. It's a hybrid bike with rigid suspension, which I'm told is better for riding on pavement.

There's a pretty tame park close to where I live, and my question is, will I do any damage to the bike riding it on dirt/trails? Would it be wise to invest in "knobby" tires and switch them out if I'm riding trails? Or am I overthinking it and I'll be fine with the standard parts on the trek 7.1fx? Yes, I know it's not ideal but I'm not looking to be a competitive mountain biker, just a way to look at some scenery and get in shape.

Any advice would be appreciated, thanks