Not if you hire a car it isnt, Took me 2 hours in a car in June, Some amazing roads to negotiate also its very straighforward and well signposted, You can also get a bus from Bodrum bus station make sure you get the direct bus and not the one that you have to change 3 times on as I did that in the past also, lol, Bus transfer time is 3 hours.

I've had the same problem in the past and had to get a flight to bodrum instead of dalaman . Its actually only about 25 kms further in driving distance from marmaris and takes around 2 hours . A private transfer will cost you about £ 120 .