transmission

My 2001 V6 ford escape has 100,200 miles on it. I found out the 2nd gear is slipping and thus needs a new transmission, about $2400. The mechanic told me it was fine to drive it for the next few weeks until I am ready to have the repair done. I am worried the engine might get damaged driving with a bad transmission. I am also wondering if it sounds worthwhile to fix a transmission on a car with over 100,000 miles that I drive a lot.
Thanks!