Agree with KoB, have a chat with Phil. He’ll have contact with guides that’d probably be willing to help, but the luggage support and accommodation being sorted was one of the best bits of the trip for me. Navigation’s not particularly difficult, but having a guide means you can ask about particular bits of trail and just follow someone down instead of having to worry about going the wrong way. The climb up to Champex-Lac is LONG, pace yourself on that one. The rest seem fairly comparable to big UK climbs, other than maybe the climb up to the Grand Col Ferret.
Altitude was noticeable on the first, third and fourth days for me, the fifth day was just a case of survival, it does get easier as the week goes on though.
I was very worried about my lack of fitness, but the climbs are long enough that I just ended up going at my own pace rather than caring how fast others were going, which makes it a shedload easier.
Only downside I can think of was that some of the portions weren’t quite big enough, but that may have been due to us being in two pretty big (12ish) groups. If you get good weather for the whole week you’ll have an absolute ball, the trails dry very quickly so it’s not a problem if there’s rain before you go. There was heavy rain the day we arrived, but by the time we went riding the next day everything was dusty again.