For a trek at this level, fitness is very important and you may have to improve yours before starting the trek. You will be walking long hikes in mountainous terrain at significantly high altitudes; weather and remoteness can also play an important role. Previous trekking experience is not essential if you are confident of your physical condition, but it is desirable.
During a strenuous trek you walk usually between 6 to 8 hours; there may be a few longer days for crossing a pass or reaching the sleeping place.
You can fill in your remaining days in Nepal with a wide range of activities. Discover some days the jungle in one of the nationals parks in the South of Nepal, go enjoying the shores of Phewa lake in Pokhara or visit the cultural sites in Kathmandu Valley.