Actually that's not what Landwehr was.
German-style system is quite complicated, but I will try to keep it simple (also, I am in no way specialist, so read my words more as general concept that concrete description of any mobilization system). Every able-bodied man between age of 17 and 45 was obliged to serve in army. At age twenty, they were called up, taken before medical commision, and if found able-bodied, they were sent to serve in Active regiment for two years. After that time, they were sent to Reserve for seven years. After that, they were sent to Landwehr regiment for ten years. After that, they can be still called for landsturm/general levy.
Now, when general mobilization is ordered, reservists are expected to go to their regiment depot. Some of them (two youngest years, I think?) are put into Active regiment ranks (while regiment exist in time of peace, it is understrength by wide margin). Other reservists with chosen officers of Active regiment create Reserve regiment. In WW1 practice that units were formed into their own Reserve divisions. At the same time, Landwehr/Territorial deployed their own regiments, which in ww1 practice also formed their own Landwehr/Territorial divisions. So basically there were five levels:
1. Active soldiers in Active regiments
2. "Young" reservists, expected to fill Active regiment to full strength
3. "Old" reservists, expected to create Reserve regiment
4. Territorial soldiers, expected to fill Territorial regiments
5. People too young, too old etc to serve in army, but who can still be called in dire need.
At the same time, landwehr regiments have their active cadre, which job is to direct regiment mobilization. Similarly, Active regiments had some personnel expected to direct mobilization of its Reserve regiment (rule of thumb: every three-battalioned Active regiment was expected to mobilize single two-battalioned Reserve regiment). Both of them also have their own non-active officers, who will be mobilised with ranks and put into command positions of newly-created subunits.
Important thing to note is that both Active and Territorial regiments were organized on territorial basis - they had to collect their "furloughed soldiers" as fast as possible after all - but they were two different organizations.
Now, what I wrote is only general concept behind system. I didn't wrote about one-year volunteers, guards, cavalry mobilization (infantry during mobilization multiply itself few times, while cavalry keep more-or-less peacetime establishment; on the other hand, reservists of cavalry are mobilized to transportation units), Ersatz divisions (made of soldeirs who never served in Active regiment, because there were more conscripts that army wanted) etc. to keep it simple. I also am in no way specialist in topic, so it is very probable that I make some mistakes (f.e. I do not really get the role of territorial soldiers).
based on:
en.wikipedia.org
fr.wikipedia.org
fr.wikipedia.org