An informed listener doesen't get tired after 20 minutes - he is entranced by the music and doesen't respond to worldly things like ''tiredness''.. Kind of.
But thanks to James for the excellent example of an attitude you should try, at all costs, to avoid.
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Anyway, Nigel (and others), as far as I can remember the first symphony is laid out in this fashion:
Introduction - Slow, pompous material in the style of a french overture
Exposition THEME 1 - energetic, extremely simple triadic theme
Exposition Modulating Bridge - nonthematic bridge which goes to g major for theme 2
Exposition THEME 2 - delightfull, also extremely simple theme
Exposition CADENCEMATERIAL - material that serves to transition back to theme 1 for an exposition repeat
here the entirety of exposition is repeated, the difference being that the cadence material doesen't transition into an exposition repeat, but a development.. The development is completely classical, filled with sequences of the main theme.
after the development is over you'll hear the exposition again, but it's slightly altered. You'll know the development is over when the main theme 1 comes back in played by the entiiire orchestra.. After that we hear the bridge again which doesen't go to g major, causing theme 2 to also be in c major!
after the exposition has been played out you'll hear a coda of moderate length that concludes the movement
Clear enough? I might edit in more details once I have the recordings available to me..
PS: The last movement is also in sonata form. Do you want to go through that one as well? And can you grasp the middle movements? The minuet of this symphony is extremely interesting for it's period...