You may certainly work by either specifying duration or entering the work for each resource and letting Project calculate duration.

You expressed that you knew how long a task took one person. How long is duration. If you said you knew a painter could paint 100 sq feet of wall in 8 hours of work -- that is work.

Use what ever method makes the most sense for you. You are correct -- if you set duration, Project will calculate work based upon:

Duration * assignment units = Work



You can specify work and Project will recalculate duration based upon:
Work/Assignment units = Work.