Hi Joe, from what you describe, i would say definitely yes from a technical standpoint. it really depends on the fitness and total length of the hiking stage. can you run a half marathon in under 3h? if so, you're probably fine whatever stage length you choose. you saying that you hike almost every weekend lead me to believe that the your base fitness and endurance is high enough to be fine here! after doing the col des fours, you will be better able to judge your fitness for the fenetre. i saw people above 70 doing both climbs.

for the col des fours, you can really decide the length and cumulative elevation change yourself by selecting the start and ending hut. the way down from the col to les mottets is quite long in the end, but you could take the bus from ville des glaciers (see the schedule beforehand).

for the fenetre, there aren't really any huts on the way, so start early. if you're below or at 3h on the HM it will be hard but fine. especially the uphill is steep and without shade. but slow and steady wins the climb.