Yes, expect plenty of "silly steep" sections in Chamonix.
We have done the bit over the back of Les Houches loads of times. There are a number of routes which take you all the way down to St Gervais, then down to the station to run back up to Les Houches to do it all again.
It's feasible to get over to Megeve this way too. Sorry, need to get the French OS style maps out to give you the names of lifts and peaks to head for etc.
Will definitely do the Brevent to Servoz next time as well as the Flegere-Vallorcine. We were hoping to do Flegere-Valorcine on the day the clouds blew in and visibility dropped to a few metres. It got very wet and all enthusiasm waned!
We once took an old lift and a old mining train up to Lac Emosson from Vallocine. That was interesting, especially riding along the damn wall which is hundreds of feet tall. The ride down to the valley floor was good fun. The lift was expensive, but you can actually drive a car up there.
If you want a tough mostly road hill climbing day (around 5000-5500ft), take the train to Montroc. Ride up to the lift station at Le Tour and get an uplift to Col De Balme, then go off the back down into Vallorcine. Join the road and then take the road up to Finhaut. There's a switchback trail that takes you over into Switzerland and way way down on the road to Martigny. You have lunch then ride all the way back up. It took us 2.5hrs to Finhaut, then another hour back into Chamonix. You can chop off the last bit of road at the summit between the two valleys by going through the railway tunnel (it's wide enough for pedestrians and trains, but you need a light). I would not recommend it because the views are great up there and the road off the top is full of nice bends (cars can't keep up with you on these). The last half hour is a blast back down to Chamonix Sud, part road, part throught the parks, forest, by the river.
There's a good few routes off the Argentiere lift too.
Ah Chamonix, so varied and so few other mountain bikers. I love it!