I want to ride a bike to work, but that is.. terrifying.

Hi, mountain bike racer here. Let me tell you a secret: I am STILL terrified of (some) downhills, yet I do them anyway. Here's the thing. Like Eyebrows upthread said, there are no rules saying you can't walk the bike wherever you don't feel safe riding it. I race Expert level in one of the most competitive mountain bike regions on the planet and you know what? I still have to hike-a-bike through shit sometimes. Go to your local bike shop. Don't just ask them about bikes, ask them if there's a good mountain biking coach or skills clinic or beginner social ride in your area. You want beginner level skills training, and you want a hardtail MTB, a decent one, not the cheapest old thing you can find. Get one with disc brakes, not rim brakes - they will help dramatically with control on steep loose descents. Get a helmet. Get a couple lessons from an experienced rider. If you lived anywhere around here I'd teach you myself. Good luck, go slow, and remember no one's keeping score. You can do this!

Could you link to some pictures of these trails? It might be that people who ride a lot will be all "eh, those are not so bad, once you've done them a couple of times you'll be fine" or maybe people will be all "yes, those are eleventh level ninja trails and you should walk them". Going down hills on a bike versus on skates: you are in a really different position and your use of the brakes is really different. I think it's much easier to brake on a bike and it's much easier to keep your balance. There are lots of hills that I would not skate down where I feel very comfortable biking.

I broke my leg on a bike when I was 10 and gave it up permanently after that; bike-riding was one of the great fears of my life until I was in my early 30s. Then about four years ago, I discovered there were paved trails about 90% of the way between my home and office, and I became obsessed with avoiding a car commute if at all possible. So I determined that I would learn to ride a bike so I could ride to work. Boy, was it scary. But you know, I pretty quickly realized that it was the idea of riding a bike that was scarier than the actual doing, especially once I had the fundamentals down. Even now, four years later, when I'm lying in bed knowing I'll ride to work the next day, I'm still a little terrified picturing myself riding down one particular hill. When I'm actually there, though, on the bike with my hands on the brakes (just in case!) - it's not scary at all. Start slow. Go try the scary parts on a weekend when you don't need to go to work right after. If you're freaked out, get off and walk the bike in the scariest parts. I still do that sometimes. Spend a few weeks riding around in unpopulated areas on safe, level surfaces to build confidence. You can do it! and I don't think you'll regret it. I certainly don't. At the very least, people don't make fun of me anymore for being scared to ride a bike!

I would not opt to ride a bike down a steep hill with lots of gravel. It is usually pretty easy to use a combination of braking and gearing to go down hills pretty slowly, but I would be really leery of miscellaneous loose materials on the trail. You're probably going to want a mountain bike if you even contemplate this. The big nubby tires deal with loose material a lot better than a typical tire does. With the hill itself, I'd be more worried about how you're going to bike home. That sounds like a lot of tedious uphill riding. Walking might be splitting the difference in terms of how much more convenient biking would be.

Without seeing the hills in question, it's hard to say whether they're "oh yeah you can take a road bike down those, just be careful" or "wtf those are technical mountain bike trails you'll die". If they are the latter, you don't want to be taking a step-through casual cruiser / heavy Dutch bike / road bike on them. You'll probably want a mountain bike of some sort, maybe not one with a suspension or anything, but something with 26" wheels and fairly wide tires to give you some grip. And you'll want to practice it, probably with some extra safety equipment (you mentioned you've done some rollerblading, so wear your 'blading stuff) so you don't hurt yourself if you do fall, on a weekend before riding to work that way. What are other people riding on these trails? Is everyone on a mountain bike? Or are there people with bikes with skinny tires and fenders and stuff riding them too? That will give you an idea of how technical they are in an objective sense, and also what kind of bike is reasonable to attempt them on. Also, when you descend a hill on a mountain bike, particularly on any sort of loose terrain, you don't want to be riding the brakes all the way down. That's how you fall. First, because the bike is inherently more stable when it's moving along at a better-than-walking pace, second because if the brakes are applied when you go over loose dirt/sand/whatever, the tire will skid, the wheel will stop rotating, and (unless you are very quick to release the brake and recover) you'll fall. You have to use the brake on a downhill judiciously, shedding speed on firm sections of trail and rolling through loose sections. The advice of walking your bike (carrying it, if it's light and the terrain is very rough) is absolutely fine. Particularly if you are just commuting to work and not wearing proper safety equipment, there's no reason to ride a descent you're not comfortable with. Go out on a weekend when you're not in a hurry and get comfortable on the hill, don't do it when you need to get to work and have your mind on other things.

There is for sure going to be an introduction to mountain biking course somewhere near you. Find one, take one. Just a day on a bike with a teacher is going to give you lots of skills and confidence. You may even find you love it. I suspect you'll find that some of those scary parts become ordinary pretty quickly.

I will only generally encourage you to pursue this, because biking to work is a huge quality-of-life improver, in my experience.

Get the right bike. Go to you local bike shop and ask about that trail. If you already have a bike, then ask anyway. Bikes are customizable - they can swap out brakes, gears, all kinds of stuff. Ask if they have a bike maintenance class - they most assuredly do. You want the class where they teach you how to do common roadside repairs on your own bike. Stuff like fixing a flat tire. Knowing how your bike works is a big confidence builder. (Living in bike loving San Diego, some guy usually comes along and changes my tire before I can get out my tire levers. But if I had to do it I could do it in about 2 minutes.) Now take your bike to that hill on a Saturday and ride the hill a bit at a time. First, ride the bottom bit. Next ride, go a little higher up and ride down. When you start to feel like it's too much, you've done enough for today. Next week, you try again and try to go a bit higher. Also, there is no shame in being smart enough to get off the bike if you encounter an area where you can't navigate safely. I got off my bike in an Ironman 70.3 because there was a tricky downhill chicane that gave me the panics. If you can't do it safely, then hop off and walk it.

It is usually pretty easy to use a combination of braking and gearing to go down hills pretty slowly, but I would be really leery of miscellaneous loose materials on the trail. Gearing has absolutely nothing to do with how fast you will go downhill on a bike. If I had a descent like the one you describe on my commute, I would want a bike with somewhat large, knobby tires. You don't need to go all the way to a mountain bike, but whatever bike you have you might want tires like http://www.nashbar.com/bikes/Product_10053_10052_554640_-1___202472. I think what you should do is ride to the bluff on a weekend, walk yourself and your bike to the bottom of it, and try it out in sections from the bottom to the top. If you don't feel safe, just stop. For your commute, you can always just walk the hill every time, as long as its not too long.