Thank you all, guys. I went out for an experimental ride today, having found you can back SPD-SL pedals off lower than "the lowest setting". About 10 miles there and back. Felt pretty good actually, lycra notwithstanding.
My cadence was pretty low, but I didn't get into granny gears at any point: small chainring all the way. But it was just a try-out, to see what it felt like.
I have bought a Garmin Edge 500 with heart rate monitor and cadence sensor. It had just come when I arrived back home.