What is a long sermon chronologically speaking? I suggest 1-10 minutes is a short sermon, 11-20 minutes is a standard length sermon, 21-25 minutes is open to description, and 26 minutes or more is a long sermon!
It would be remiss of me not to make some suggestions as to how to preach a great long sermon. My 'thing' about long sermons has never been that they should not be preached; it has been that few preachers seem capable of preaching them well. So, what can we say? The following list of suggestions is not intended to be exhaustive.
(1) Expound a passage of Scripture rather than a topic or theme.
(2) Have three main headings and, if possible, subsume those three main headings into one memorable theme.
(3) Tell stories at appropriate points during the sermon, not only to illustrate the points you are making, but also to sustain interest and engagement with what you are saying.
(4) Marry your exposition of Scripture with commentary on daily life. In a word, be 'relevant'. Make daily life in Palestine, or Paul's concerns about Corinth, or Isaiah's preaching about Jerusalem connect to the daily lives of your hearers.
(5) Use some techniques to reinforce your message or messages. To give one instance, as you introduce point 3 you might restate points 1 and 2. In your prayer at the close of the sermon you might sum up the message.
(6) Be self-critical in your preparation. Revise and re-revise. It is very important to preach one sermon, not two or three. You will undermine the advantages of a long sermon if your 25+ minutes consists of a great 15 minute sermon and 10+ minutes of padding. (And, if that is the assessment of what you have got in your draft, it might be fruitful to preach a great 15 minute sermon rather than spend more time changing the 10+minutes of padding.
(7) Think about your audience and what works with them. It may work for you to spend 5-10 minutes working your way up to your main message, but it can be terribly distracting for your hearers (indicated, in all likelihood, by their restlessness and inattention through this period). Your opening joke may help settle your nerves, it may be enjoyed by the hearers, but what does it achieve?
(8) (also 1!!) Pray.