Nick - can't help you re the protein on board, as I don't need it for my distance, and my carry on is just full with my trainers and running clothes!
However, we've just booked today to go to Los Pocillos (which is just above you) at the end of November as there's the Lanzarote Marathon festival on Sunday 08 December, and I'm hoping to do the half marathon.
It starts at the far end of Costa Teguise and runs down the coast, past the airport path and down to where you are. Have a look on their web page, as there's several events on the day and a little video showing some clips. Not quite clear on whether out & back routes and mileage on there. I'll be entering shortly before we go so will have fuller detail when I do this.
However, we're back a couple of days after the event, and if you want to PM me then I will email over to you everything I've got showing clearer detail, routes and mileage from my Garmin if it helps you plan for next year.
Also, we've been to Costa Teguise before and it's good for runners, have been running in Playa Blanca, and a friend of mine who visits PDC regularly says she sees loads of runners along promenade going up to airport as it's a good flat run, so I don't think you'll be short of decent runs to keep up that mileage!
Maybe they're strapped for cash, as you say they've got 2 youngsters, and this could just be pushing their finances a bit too much, causing her to be so abrupt with your dad? Maybe your brother didn't want to have to deal with this issue himself so that's why she stepped in?
I hope the above link works. Please tell me if I'm reading it wrongly, but it does refer to running 3 miles in 15 minutes rather than walking it in 38? Wish I could run that fast, or walk that speed, sure a lot of you can.