Hi
I went last year and there are a few different routes available. The campsite we were staying in had a book that outlined the various routes. Also there are lots of walking trails you can ride if you’re a naughty cyclist.
I did a ride up to Col de Vallette and it was beautiful etc. but we didnt take an OS map and got very lost at one point… So I’d advise one.
Another thing is that 28km doesnt sound like much, but over alpine terrain it was a bit of shocker and took all day.
Jack.
[Edit]
we drove to the village beneath the col de vallette that the book recommended.
Loads more around if you ask, go looking. The bike park was boneshakingly rutted on my Process153.