Not done the Kintyre Way race (yet) but did my first ultra this year at the Speyside Way Race. Enjoyed it from start to finish despite suffering from leg cramps from about 20 miles on. I prevaricated about entering for 3 years but was glad I took the plunge. If you've done 4 marathons you should be fine. It's been 10 years since I ran my last marathon though I regularly run long hill races. Three 25 mile long runs in the months before the race and 60mpw got me through. Anyone thinking about ultras should have a look at Andy Mouncey's book on ultras, lots of very good advice. I particularly liked "90% of ultra running is psychological. The other 10% is all in your head."
I have been tripped up by this belief that the young 'uns aren't as fit as us old hands.
I managed to persuade one of my recent trainees (who had a good 10 years on me) into running a particularly tough and brutal race. He'd only been running a couple of years on and off and I thought that as an old hand with nearly 30 years experience behind me that I would "wipe the floor" with him.
Sadly he passed me at half way and finished 15 minutes ahead of me.
I learned my lesson - don't start off too fast and don't assume experience will win out in the long run.