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Let the number of miles Phoenix hiked in each day be $a$, $b$, $c$, and $d$. We have the equations $$\begin{array}{rl}a+b& =26\\ (b+c)/2=12\Rightarrow b+c& =24\\ c+d& =28\\ a+c& =22\end{array}$$ Adding the first two equations gives $a+2b+c=50$. Subtracting the fourth equation from this last equation, we have $2b=28$, or $b=14$. Plugging this value of $b$ into the first given equation to solve for $a$, we find that $a=12$. Plugging this value of $a$ into the fourth given equation to solve for $c$, we find that $c=10$. Finally, plugging $c$ into the third equation gives $d=18$. Thus, the entire trail was $a+b+c+d=12+14+10+18=\boxed{54}$ miles long.

Of course, you could also have realized that the total for the first two days was 26 miles and the total for the last two days was 28 miles, meaning that the total for all four days is $26+28=\boxed{54}$ miles.