For me, I can run and I can walk, and bicycle riding falls in between the two in terms of difficulty. So for me to run a marathon would be just about impossible. For me to walk it, no problem, just give me 8-13 hours or so and I'll git 'er done. But if I ride my bike in a century, I'm generally working harder than I am when walking.
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"be careful this rando stuff is addictive and dan's the 'pusher'."