The two sides meet at the Millennium Stadium in the play-off final, knowing that only one of them will be in League One next season.
And Alexander reckons the long-term security at the club will work in his side's favour.
He said: "I'm delighted to have signed a new contract at Lincoln and hopefully I'll have two reasons to celebrate.
"It wasn't a difficult decision to make, but I had to make sure I did what was best for me and my family.
"We can now push this club forward. Firstly into League One - which is my main priority - and then into the Championship.
"Hopefully the majority of the fans and players will be happy at me signing this deal."
Last year Lincoln lost out in the semi-finals of the play-offs to Huddersfield, and were hammered in the final by Bournemouth - having beaten the Iron in the semis - the year before.
Alexander knows his side are the underdogs, but is confident they can still win through.
He added: "They [Southend] are a strong and aggressive side - very much like us in a way.
"I think they have weaknesses though and we'll be looking to exploit them. We are focused and are going to enjoy the day."
The Imps have sold over 10,000 tickets for their clash, and have commisioned a special train taking supporters to Cardiff for the final.
Meanwhile, Southend have been quietly building up for the final this week, and boss Steve Tilson is treating tomorrow's game like any other.
He said: "We won't be staying at a posh hotel and we aren't having suits or kit.
"Everything has been kept low key as we can by staying in a run of the mill hotel and we are going in tracksuits."
This is their second experience of the Millennium Stadium this season, following their defeat in extra time of the LDV Vans Final against Wrexham.
Southend are hoping to take around 15,000 supporters to Cardiff, but so far just over 10,000 tickets have been sold.
Shrimpers fans have got in the spirit of the occasion by bringing out a 'Seasider's Song', which celebrity fan Terry Alderton has made in collaberation with Bell and Spurling.