Good advice from Runnersweb.com

"I made it through my workout; therefore I ate and drank enough. "

There is a big difference between what is optimal and what you can get by on.
I often see athletes gravitate towards the latter. Dehydration raises
heart rate and lowers endurance. Glycogen depletion leaves you with
little energy for high intensity work. Not eating or drinking enough
degrades your performance. You may be able to complete the work out,
but you could have pushed harder, gone faster, and accomplished more if
you had followed a good fueling and hydration plan. The longer your
training session, the more important this becomes.

"Close enough is good enough."

Training requires precision. For example,
the difference between a good aerobic capacity workout and a
non-productive one can be a few heartbeats and seconds. In order for
adaptation to occur, the body has to have a new stress level placed on
it. This means breaking new ground. If you apply the same level of
stress, or less, you will not get faster. The nearer you are to your
goal race, and as work out intensity goes up, the more important this
becomes. Athletes are often surprised when I tell them their workout
did not accomplish much because they were slightly below or even above
where they should have been. They may have worked hard and were very
fatigued, but did not have that last little push that to take them to
the next level.

Xeno Muller, Olympic gold and silver medalist, indoor rowing, rowing technique.