A bus service is available from Mashhad in Iran, buses are supposed to leave a 7 AM from the bus terminal but departure times are flexible, be there early. Arrival at the border is around noon and in Herat a bit after 3 PM. Border procedures are relatively straightforward.
Overland travel by car can be both time-consuming and dangerous. The road from Kandahar has been rebuilt but is extremely dangerous as it passes through Helmand and Farah, both which are active war zones. The roads from Iran and Turkmenistan are both in good shape, the later one being tarred. However, expect a few craters here and there. There are occasional security incidents on the road from Turkmenistan as trouble spills over from Badghis province which is unstable. Get up-to-date advice before attempting this route. The A76 highway connects to Mazar-e Sharif via Maimana. The upgrade of this road is not yet complete, largely due to the kidnapping of the construction team in April 2009. This route is not recommended.