This belief seems to be common: "We started the game well, then we got caned in the last 20; therefore, there was something we were doing in the first 60 that we were not doing in the last 20." Germ, for example, happens to believe that we strayed from our game plan in the last 20) - this is not necessarily true, though. I don't believe it; I actually think the primary reason we were leading was a lopsided penalty count, ie, luck.
There are many other possible interpretations too: I think one valid reason why we tend to score more points in the first 40 is that we rely a lot on our forwards charging onto the ball full steam. When they lose the ability to do this, the "up the guts" game plan that we use, no, RELY upon, starts to go pear shaped. Now, this has little to do with fitness - very few forwards, if any, have been able to run as hard after 50-80 minutes of football as they can at the first minute. It is a GAME PLAN problem, because it involveds putting too many eggs in one basket. (I'm not suggesting that this is the case, but it deserves consideration. Simplified views such as "we lack fitness", "we slacked of mentally and lost concentration" or "there was no pride in the jersey" can only explain so much imo)
