About 600 km from Varanasi to Mathura–Vrindavan. Overnight train is usual. This is the distance/tour explainer.
Varanasi to Vrindavan / Mathura distance is about 600 km. Kashi to Vrindavan is the same. Overnight train is the usual plan. Road is 10–12 hours. Prem Mandir is in Vrindavan (not next to Kashi Vishwanath).
Book the Braj package: Varanasi to Mathura & Vrindavan tour. Combine with Ayodhya on a longer trip.
Temples: Krishna Janmabhoomi (Mathura), Banke Bihari, Prem Mandir, ISKCON, Yamuna aarti. Plan 2 days / 1 night minimum in Braj.