Tired Engine Signs?

[MikeIIDC@domain.elided]

What are the indications when an engine is getting "tired"?  I had never
considered this possibility with my engine, because once it warms up, it
runs terrific.  However, after driving it slightly over 400 miles
yesterday, I began to wonder some things.  For one, the thing is very
slow to pick up from a dead stop.  This is worst when it is cold, but
even when warm, it is Volkswagen-esque in its 0-60 time (And I mean REAL
VW-the original Beetle, not all of that new-fangled stuff they have out
now!).  When on the freeway, it iwll not complain at all about moving me
along at 75 MPH or so, and is pretty good accelerating from about
50-80.  However, it does really lag from lower speeds and stop up to
highway speed.  I was also driving on lots of hills yesterday, and
noticed that it wouldn't always maintain speed going up hill.  I started
getting paranoid about this, but then noticed that many new pick-ups
were not exactly leaving me behind!  I kind of forgot about it, but in
thinking about it a little bit more today, I remembered that my '68
Travelall with the same engine (345) was excellent at accelerating (for
what it is!) and even better on hills.  I actually wrote a short story
about that truck and talked about how its absolute top speed was 75,
whether going downhill with the wind at your back or going uphill into a
monsoon with three trailers.  It seemed that nothing ever slowed it, and
that was even before I rebuilt it.
I know that ultimately I have to do a compression test to see what's
going on, but do these sound like the signs of a tired old girl?  I was
thinking that if I couldn't get the 304 in my 800 running better, I
would simply drop the 345 in, but if this 345 is not strong, why bother?

I must admit, this was supposed to be a parts vehicle, and since I
wrecked my Travelall and had to get this running, no mechanic has
touched it except me (I know, you're all surprised that it's running at
all!).  I always have assumed that the slow acceleration and stuff are
because I don't really know how to dial it in.  However, this wouldn't
really explain the times that it does run so good.
And what is with the whole "warming up" thing that engines do?  If all
they are doing is dumping fuel into a cylinder and igniting it, why
should it do this better after a few minutes of running?

Thanks!

Michael