At Odyssee Montagne my guide company, this is the main question that mostly clients asking us. Mont Blanc normal route is a beautifull climb but to enjoy it it require a very good physical condition.
We use to tell people to start their training few month prior their climb, by running or cycling in different terrain. Training endurance is essential. Two main factors are crucial!
You must run or cycle a minimum of one hour. Your heart and your muscles need at least 20 minutes to warm up, so you will train efficiently for 40 minutes. You'd better run or cycle once a week 1 hour minimum instead of 3 time a week 30 mn!
Second factor: You must varied your routine, sometimes flat, sometimes hilly, otherwise you get use to the same one and you always use the same muscles.
Running in indoor fitness center is not as efficient as running outside on natural trails. When you run outside your feet and ankles are working and you improve your proprioception, which is really important in mountain. You improve your balance.
Climbing Mont Blanc involves walking on rough trails and terrain, and using mountaineering boots. Many people have never used stiff mountaineering boots and when they use them for the first time, they are completely lost! It change the volume of your feet, your balance and therefore you loose much energy.Try to walk with these kind of boots before your coming and in mountain if you can! It will be the best training.
If you want to evaluate your fitness level: A good speed is the ability to walk/ascend 400m in 1 hour during 3 hours minimum.
If you used to scrambling or rock climbing it's always much better as the way between the Tete Rousse Hut and the Gouter Hut is easy scrambling, grade 1 or 2.
If you want to ask me questions or advices, feel free at firstname.lastname@example.org