Driving is more exhausting than biking

Tom or Mary said:

Last Monday I drove from Cleveland to Pittsburgh (150 miles), and I left at
7am and arrived in Pittsburgh at 4pm, having stopped at various places along
the way. I was totally exhausted. Next day I rode my bike thirty miles, then
I mowed my lawn (1hr) with a nonpowered lawnmower, and I felt as if I had
not done any physical exercise. Why does driving leave feeling so exhausted,
but physical exercise doesn't?

My first guess: Lousy seat and/or seating position in the car.

My cargo van's OEM seats would wear me out in four hours. The
replacements I installed (scavenged from a wrecked Dodge Caravan) are
good enough that I can drive for five hours nonstop, and 15 hours in a
day's travel, and not feel worn out at all.
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