Munnar is a small hill station located in the Idukki district of Kerala. It is located at the confluence of the rivers Mudhirapuzha, Nallathanni, and Kundaly. Munnar can be reached from Mumbai by road, trains, and flights.
The total distance from Mumbai to Munnar is roughly 1,500 km and the journey can be completed in half a days time. If one wishes to go via the Ernakulam route, then it would be shorter for them, and the total distance would be cut short to 1,320 km. There are direct flights from Mumbai to Kochi (Ernakulam) and they take 2 hours to complete the journey. From there you, you can travel to Munnar via car. Kochi to Munnar is just 130 km, and the journey can be completed in 2-3 hours of time. The entire journey of 1300 km can be covered in max. 5 hours.
There are also overnight trains running from Mumbai to Kochi. Once you reach Kochi via train, you can make a journey to Munnar by car and reach your destination in just 3 hours. However, the journey by train takes a lot more time than by flight, so it preferable to take a flight instead of a train.
One can also travel by the Mumbai-Trichur route, which is the second shortest route from Mumbai to Coorg, with a total distance from 1396 km. There are overnight trains running from Mumbai to Trichur. Once you reach Trichur, you can travel by car to Munnar, and cover the last 150 km in 3 hours time.