If I succeed in obtaining the address from her, we can go there at once, taking Mrs. Vandemeyer with us if necessary.
Once I get the address, we can go there in about 3 weeks. 
mnli hypothesis: Once I get the address, we can go there in about 3 weeks.  premise: If I succeed in obtaining the address from her, we can go there at once, taking Mrs. Vandemeyer with us if necessary.