in the saddle指骑马。
saddle up =saddle v.给(马等)备鞍；给...装上马鞍。=put a saddle on a horse。

【例句】
Why don't we saddle a couple of horses and go for a ride?
我们何不找几匹马套上马鞍出去遛遛？

Three weeks after the accident he was back in the saddle.
出事后三个星期，他就又骑上马了。