>>2051115
Gears are the bike's transmission and transmissions are just spinning levers. A low gear ratio creates a long lever. Easy to move, but the end of the lever moves only a little bit compared to the one you push on. A high gear ratio is hard to push, but moves the end of lever quickly. The pulley system of the chain allows the lever to made small and spinny instead of an actual big lever. The end of the lever on the bike is the wheel.

>bike
How rough is the planned gravel and dirt? If you need a mountain bike, that works just as well in the city. Otherwise, just get a generic commuter bike. 4-6 miles isn't really anything on a bike, so it doesn't really matter what kind you buy here. If you're concerned about grip on the commuter, then just put gravel tires on it.

>winter
You haven't said anything about how rough these trails are. Paved foot paths are miles different from single track MTB trail. If it's just cold, then winter tires with winter rubber will do. If it's snowy, then you're going to want a fat snow tire and maybe even studs if the trail is off road.

>fitness
Pedal hard for a long time. you should be breathing just as hard as if you were running if you're trying to use it for fitness. Having a goal like you r 8 miles is good.

>seat
Seats are like shoes. There's a billion different ways it can fit and everyone has a specific preference. Numb penis is typically from the perenium pressing against the seat. Seats with cut-outs in this area are made to address that. You may also be badly positioned on the bike and that's causing your pelvis to rotate forward and put pressure there instead of on your sit bones. Hard to tell without a picture of you on the bike/ bike seat. Don't be afraid to bring it up to the guy at the store, it's a common problem.