Did this one on Monday, been out a few times since - I live on Arran so it’s my local stomping ground
The String is a helluva climb, especially without a proper warmup first - still, you got the hardest part of the day done straight away! The south of the island is deceptively hilly (like you say, up down up down up down all the time!) and very dispiriting if you can’t get a rhythm going - the north loop has one big climb before/after Lochranza but is mostly flat apart from that. I live in the south so it’s perfect training terrain - just back from a sprints & hills session in time for the TdF highlights 