How to fix a sluggish bike? I just bought a used mountain bike, and it seems to be slow.  Pedaling seems like very hard work, and at a normal casual pedaling rate, I can only reach 4-5mph. If I pedal like I'm being chased by a lion I can get up to 10-11mph for a few seconds.

For reference, I can run a 6-minute mile, so I can run faster than that.

I'm thinking it's probably the bike, because the chain looks dirty and the wheels make scraping sounds as it moves.  (I don't know if it's scraping against the brakes or something needs oil, etc.)

How can I fix this?
Since it's a new-to-you secondhand bike, for safety's sake as well as to deal with the sluggishness I recommend you take it to a bike shop for a complete overhaul. I'll bet you'll be astounded at the difference when they're done.