When I was a student, we laboriously filled out flight logs and worked out
each leg with an E6B. What stupidity.

Obviously, the modern way (and a perfectly reasonable way) is to let a
computer crunch the numbers for you. But, you still should be able to do
an entire flight plan with nothing more than a chart, a plotter, and an E6B
in under 5 minutes.

Measure the total overall distance of your route. In these days of
GPS-direct, that usually means a single straight line. If you're flying
VOR to VOR, it's really easy to just add up the distances marked on an IFR
en-route chart.

Next, get a magnetic course. You could do this with a plotter, or just
look at a VOR rose and make a reasonable guess (if you get it to within 10
degrees, you're fine).

Next, look at the winds aloft forecasts along your route of flight and
planned altitude. Take a WAG at an average speed and direction. Do NOT do
any math. Just average them in your head. If you spend more than 30
seconds on this, you're working too hard.

Now, work one single E6B wind triangle problem to come up with an average
GS for the whole flight. Flip the E6B over and work one single
time-speed-distance problem to come up with a ETE.

That's it, you're done. Lots of guessing and rough averaging, but that's
really all the problem deserves.