I reckon the race weight guides are bollocks. They give me a BMI of 17ish. I've been that light, and was bonking walking 100yrds to the letter box. I certainly wasn't running!
I know nothing about achilles problems, but would always advise consulting a physio/sports medicine bod rather than your GP. Standard GP advice for any running injury is stop running. Unhelpful, although less dangerous than advising starving yourself til you break down.
(5'5" and 56kg. Ultra runner, although quite how I get my vast weight up the hills I have no idea. And there's millionaires shortbread in the kitchen.)
I'm going to say the opposite of Shawk, and suggest finding something you really like. I ran the SDW50 on hummus sandwiches and M&S millionaires shortbread and fridgecake bites. Ohh, and an apple. I love the M&S bites, but they're expensive, and thus a real treat. I'm more likely to eat if its posh yummy chocolate goodness than something forced down for the sake of calories.
I set the watch to bleep every 4 miles, both as a pace guide (evens out the hills!) and as a reminder to eat if I haven't already.
Thinking of trying cheese straws next time. I love cheese straws. Or bread with goats cheese. I wonder if its possible to fit a baguette is a race vest?
My first thought, even before I read it in your post, was "sugar overload". Sweetness fatigue means nothing to me (sorry, pmo). Depending on the context "sick of sugar" or "tired of sweet foods" might work.