You can either use your hands as a fulcrum, or a thin stick to place between your legs, or do a search for ‘whale* stretch*’.
Sure 40 minutes is a lot of time. You are not supposed to do a singles 20 minutes set of each, tough, you can break in, say, 10 x 2 minutes. I think the total stretching time is the most important thing. Neither I see why 20 minutes would be a ‘magic number’ - 15 or ten minutes could be just as much effective than 20, but this is not my routine, though. ;)
I have never been able to do more than 20 minutes manual stretch over the course of the day, as a side note. This routine, I would consider it very demanding.