It's true! Practice is the way...
I have only been playing double pedal 'seriously' (after a few false starts) for a year or so and up until recently I have only been using it in fills, but I just started working on playing 16ths with feet as part of the beat.
Mr metronome is my friend here, he keeps me honest.
What worked for me (so far) is:
Choose a tempo that you can play 16ths comfortably at, then play 16ths at that speed for a few minutes, concentrating on even strokes. If you get bored easily just doing this (which I do), you can add a basic beat (Ride/Snare or HH/Snare) with your hands so it feels more 'real', but maintain your focus on evenness between the feet.
If you get cramp in your shins, or if you can't keep it even, or if you can't keep going for a few minutes, then slow the tempo down a little until you can. For me the speed where that worked for 16ths was 110bpm. Not very fast, but a starting point. If it's working you may feel
just a little bit of fatigue in your shins after a minute or two. That's your muscles starting to develop
.
After a couple of weeks of doing that for a few minutes each practice (and I don't practice EVERY day), I can sustain it at 130 bpm. Again, not fast by the standards of some on this forum, but it is genuine progress, and the best part is that ALL my double pedal playing is better as a result!
Anyway, this worked for me. Maybe it'll work for you too.
Cheers!