I think my issue is the free-wheel being gunked up. This is what my bike-friend told me; he lubed it for me a while ago but told me it was just a band-aid fix and that I really needed to take it apart and re-grease. After a few weeks, my pedals once again move when I push the bike backwards.

I want to learn more about my bike, so I want to do it myself. I'll be going to a local bike DIY place, where there will be staff to help.

Is is correct to assume this job should take no longer than a few hours? And that the chances of me breaking something or needing a new part are minimal? I have an older bike, and the place doesn't really sell parts beyond the very basic. Also if I can't get my bike apart & back together in less than 3 hours I'll be screwed.