An Up-Down Walk

A sequence $a_1, a_2, \ldots, a_{21}$ satisfies $|a_{n+1} - a_n| = 1$ for $n = 1, \ldots, 20$, and $a_1, a_7, a_{21}$ form a geometric sequence. If $a_1 = 1$ and $a_{21} = 9$, find the number of such sequences.