You can max out the rails, it's okay. The only thing you lose is the ability to adjust the saddle more in whatever maxed out direction. I suppose it'll be a bit weaker, maybe, but for literally decades I adjusted my saddle by sliding it forward all the way and seeing if it ended up in the right place.
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"...during the Lance years, being fit became the No. 1 thing. Totally the only thing. It’s a big part of what we do, but fitness is not the only thing. There’s skills, there’s tactics … there’s all kinds of stuff..." Tim Johnson