Hello Sue
Great to hear you're getting back into walking and good luck with the first venture. If you are still based in Carnoustie, Brian's suggestion is a really good one, and I say this even though we don't have a guidebook for it!
If you've already done the FCP, another suggestion is a section of
Moray Coast Trail which is largely flat and has stunning coastal scenery. For an easy, spectacular section I'd suggest start at Lossiemouth or Kingston and walk to the end at Cullen with an overnight at Buckie (overall distance up to 23 miles but you could shorten that by starting east of Lossie). Your route would take in beach walking, crossing the magnificent Garmouth Viaduct, views of skerries and stacks and the wildlife of Spey Bay. Near its end you pass by the iconic Bow Fiddle Rock at Portknockie.
Although our new guidebook isn't actually printed yet, it's just about ready for repro and if you are tempted by this walk, we could support you with a PDF of all the relevant bits.
The FCP is handier for you, but the MCT would be more of an adventure

, so it depends what you are looking for. Good luck whatever you decide!

Oh, and on your original idea, Kirkmichael to Spittal is only 8 miles and although the ascent to the Lairig Gate takes you over 2000 ft, the terrain is good and you can break the hike at Upper Lunch Hut. So it doesn't sound over-ambitious but you will know best what is realistic.