La Marmotte route advice

We did it this year in July, riding from Alp d'Huez over the Croix de Fer into Saint Sorlin d'Arvres rather than to the top of the Glandon.  There is a nice artisan bakery on the left at the bottom of Saint Sorlin main street

We didn't go crazy, stopped in Valloire for a decent lunch and we were out of the appartment for the best part of 11 to 12 hours, with stops in Saint Sorlin, top of the Telegraph and then Valloire for lunch.  It was that hot that I was going through two bottles of water between every inhabited point - that will be your main issue, keeping stocked up on water.

Riding in the Alps is different to the UK.  Destroy yourself on the first climb and you will have a miserable day.

So, so long as you ride within yourself and pace it sensibly then its "only" 3 big climbs plus then you'll get up the Alp at the end even if you're on your hands and knees won't you!  

I did it in 2015. D'Huez was a crucible at the end. Someone from our party had to go to the medical tent with heat exhaustion. Another passed out on the Croix de Fer (road closures deviated the normal route).

My first words to my wife were "don't let me do anything like that again!"  Woke up the next morning and said "next year I'm going to do even better".

It's a hard day in the saddle for sure. There were people who knew they were going to slog it out long and slow. It not about size of the dog in the fight, but rather the size of the fight in the dog....guts and determination are what get you through. Do it and prove something to yourself.

You'll be fine, I rode it this year and I'm a fat little hobbit! Weather hit I think 28c on the day at times, but was markedly cooler for most of the day.

Gasman is right though in 2015 the heat was apparently awful and I think a lot of people didn't even start, never mind finsh.  

The only thing to worry about is pacing on the climbs...  just 1mph too much and you can blow up really easy.  

Traffic is sparse, drivers are used to bikes on the roads and the few KM's of highway on the route has a hard shoulder (sort of thing) which is used for cycling in.

Personally, I wouldn't do the whole ride without it being a 'sportive' or organised ride. It'd be far 'nicer' to cherry pick the course highlights or tackle the Telegraph and Galiber on one day, Alpe d'Huez time trial in the morning of another, then at the top follow it over the back of the alpe to find the lakes, and then tackle the Croix de Fer another day. 

You'll be fine. I rode ithe event in July 2015 and the temp was 44C on the final climb!  If it's very hot just adjust you pace accordingly and use some Dioralyte in addition to the usual sports electrolyte drink (it has a lot more potassium).

can't really comment on the route but one factor you need to consider is the heat in the Alps in August. Completing the Fred Whitton is one thing in the Cumbrian weather but it's a wholly different prospect in 30 + degrees and minimal shelter from the sun.

Good luck anyway