According to Bala Kanda, after elimination of Subahu and sparing the life of Mareecha by throwing him into Ocean, Sri Rama along with Sage Viswamitra reached the Northern Banks of Ganga and from there he reached the city of Vishala. Thereafter, they reached Mithila, the city of Janaka.
इति उक्त्वा मुनिशार्दूलः कौशिकः स तपोधनः |
उत्तराम् दिशम् उद्दिश्य प्रस्थातुम् उपचक्रमे || १-३१-१६
Thus saying farewell to woodland-deities that descendent of Kaushika, namely Vishvamitra, the saint whose wealth is his ascesis started to journey on aiming to reach the northern side of River Ganga.
Sage Valmiki did not narrate explicitly the distances travelled by them for reaching Mithila (at least in Yojanas).
We don't know whether the geographical locations, names of towns of that era remained the same even now also.
Some believe that Nashik is the place, where Surpanakha was disfigured, and some others feel it is near Bhadrachalam, Telangana.
Similarly, the case with the city of Janaka, Mithila, the birth place ofor Sita. Everyone according to one's perception.
2nd part of the question where did Sita stay during pregnancy?
Sita's pregnancy was mentioned in Uttar Kanda.
It is to be remembered that Uttar Kanda is an interpolated story.
Ramayana ends with coronation of Sri Rama as the king of Kosala Kingdom.