Hi Rachel, an important factor for deciding blocks based on your activity being two long hikes a week would be how much of a strength element is included in the hikes. It's not only the aerobic activity itself that drives your protein requirement, but also how much muscle repair will take place in the recovery during the day(s) after you hike. Regardless of the length of the hikes or the condition of your legs, the more of a strength element that’s involved, the more protein you’d need. See how it goes with the 13 blocks and monitor LBM over time. You'd want it to stay the same or increase, not decrease. Don’t be afraid to increase to 14 blocks and/or add more monounsaturated fat if needed to maintain peak energy. It's very important that you eat enough protein to maintain your muscle mass, and it's also important not to eat more food than you really need.
Re calculators, the one on this site is a very good tool for determining your protein/ block needs. The protein recommendation in your calculator result is not written in stone. One important feature of the Zone diet is your ability to adjust it to suit your needs. Regardless of what method you choose to determine LBM, what's important will be to use the same method consistently over time for valid comparisons of LBM.
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