A little diversion - when thinking of Scottish Islands does Kintyre get overlooked? It's very nearly an island, and has much of the character of one. The obvious loop down to Campbeltown is about 64 miles, a little more if you start at Tarbert, so it's probably a little long for a first attempt, but one to bear in mind. When I was there I didn't go beyond Campbeltown, but there are additional attractive-looking circular routes further south.
What you do get is a wonderful contrast, with much of the inlet-free west coast being open to the Atlantic, and the east being far more sheltered and overlooking Arran, just a few miles away. When I rode that circuit I was a little concerned at the steepness of the hills on the eastern side, as indicated by the OS map, but found that the very steep sections were quite short and with the ever-present fantasic views the climbing went almost unnoticed. That's what I remember anyway. I'm probably romanticising a bit.