They travel to face Northampton on Sunday, before the return leg at Roots Hall next Saturday.
Lincoln and Macclesfield will do battle in the other play-off semi, they play their first leg on Saturday.
Southend put up a strong challenge for automatic promotion, and boss Tilson insists their last day drop down to fourth - after failing to beat the Cods at Blunder Park - counts for nothing now.
He said: "Obviously we are all disappointed not to go up automatically, but I
felt we had enough chances to win the game twice over.
"We've got to pick ourselves up and get ready for two tough ties against
Northampton in the play-offs.
"We'll remain positive in training because we don't want to waste all our
hard work this season."