Entries with an M means that we did not have any operating hours associated with the data point (i.e., no velocity information); therefore, it is not possible to convert these entries from Miles to Operating Hours.
Often for reliability analyses I have to make engineering estimates.
In this case if you have miles just divide operating miles by average speed (make a conservative estimate) and you obtain operating hours.
An educated guess is better than nothing.
But any system should be subjected to testing to ensure it will meeting operating environments and duration.