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You can use Fibonacci numbers (though obviously they round to integers)
2 m = 3 km
3 m = 5 km
5 m = 8 km
8 m = 13 km
13 m = 21 km
etc.which gives e.g. 26 miles = 42 km which is close enough for most. Perhaps easier to do addition than multiplication by 1.61 in your head.
Edit: For maths nerds this is because the golden ratio is numerically very close to the mile-to-km conversion factor, and the limit of the ratio of successive terms in the Fibonacci sequence is the golden ratio. Good to 0.5% ignoring the integer rounding.
frankenbike
5awyr
PhilPub
For reasonable accuracy (off the top of my head), 1 mile = 1.61 km, 0r 1km = 0.62 miles. So 10 miles = 16 km / 10km = 6.2 miles.
Some rougher approximations which come in handy for on-the-fly estimates:
3 miles = (a little under) 5km
5 miles = 8km
Commit these to memory and you get the hang of things pretty quickly.
(Not to mention 26.2 miles = 42.195 km, always a handy one to remember!)
If your Garmin 235 is like my 220, you can pre-set intervals/rest periods of either distance or time, under :
Training>Intervals>Edit