Do you mean you actually used it for like 21-24 hours (I understood there was like 3 days from last charge?) and battery only went down to 40% during that?
Are you sure you used it on battery all that time? And it is standard MBA M1 with standard battery that comes with it or did yours come with a car battery?
Because I have MBA M1 and it can run on battery
in actual use for something like 10-14 hours at most (I mean actual use, not that I let it sleep most of the time).
Battery use per hour in mine is around 7-10%, there is no way I could use it with only 2,5% battery use per hour in any way (maybe if turn display brightness to 0% it could be remotely possible but hard to use).