I want a bike that is fast on the road but can also go offroad. By offroad I mean mostly gravel and a little dirt but no mud or very little. Cyclocross bikes caught my interest but they seem expensive. Can I buy a road bike and change a few components to make it more suitable for offroad? About 100km is as far as I'll probably ever go. I'm not a cyclist but I am a runner. I'm worried that the gears are too big or how should I say it, I'm afraid I won't be able to go uphill with a road/cyclocross/gravel bike.
It sounds like what you want is a gravel or adventure bike.
There is not a distinct category, and different manufacturers use different names, but the basic attributes you are looking for are:
- Road bike style frame, ISO 622 rims, drop bars
- Clearance for up to about 40mm tires
- Disc, linear pull (V) or cantilever brakes
- Slacker steering geometry
- More relaxed body position
There is a blurry line between this type of bike and a cyclocross bike. True cyclocross bikes have recently moved to 1x11 drivetrains. Unless you live in a very flat area you should opt for a bike with a double front chainring.
There are offerings from many manufacturers with aluminum and carbon fiber frames (and some steel) at a variety of prices. You can also look for a used bike.
Gearing can be changed, though sometimes it's hard/expensive. Some adventure road bikes and almost all tourers have a much wider range of gears, but tend to be heavier than road bikes.
The available width for tyres on a road bike can't be changed, and will limit your off-road use. Gravel is very variable; some is hard on road tyres while in other areas you need a bit of width in your tyres so you don't sink in.
Starting from a cyclocross or adventure road bike is a popular approach for getting such a bike. I didn't do that because I wanted the long chainstays of a tourer (panniers + big feet). For a maximum of 100km and not racing, any of these bikes will be fine. If you're worried about speed, choose your tyres for the type of ride. Be warned though, you might think now that 100km is your maximum, but you might change your mind. A year ago that sounded like a lot to me, and I decided to buy my tourer. I did 230km yesterday.