Variance in poker evens out over months, not a single session. A competent but not great player can definitely beat a pro in a single session.
A competent but not great player has a chance of beating a pro in one session, but it's definitely pretty slim.
For every tournament you make a big score, you crash out of 5 and get min payout on a few more.
Many of the biggest names in poker have been the biggest losers in a given calendar period. For example, Gus Hansen and Daniel Negreanu are two of the biggest winners of all time yet also have some of the biggest documented losses...
As for how long a session is? 8 hours. I made a living from poker for a few years.