You session time is the total amount of time where the user is connected to a service/server throughout your test. The duration time is the length of the test run. So if you are doing a simple login/logout excercise, then you would have for instance:-
Vuser 1 - session busy time as 8.837 (s)
Vuser 2 - session busy time as 6.032 (s) ... etc
Duration 1 hr.
So in essence you can have multiple sessions within your 1 hour test.
What you need to do is run your script as a test (not 'Try Script') and then you will get the 20 VUser data from the overview report.