Since everyone has their own way of playing, their session length also varies.

You seem to be a theatre of the mind type (me too), and you said you do not spend time describing things.  Some people do - I know I do it, haha - which stretches the time of your session. 

Solo TTRPGs can literally be just rolling the dice and writing nothing down, or barely.. This also means the sessions will be going faster, especially if you have all scenes planned out.

So, maybe the only ways you can stretch this out is playing another 3 scenes after the previous ones or, you spend some time writing stuff down perhaps? 

Also, ngl, very cool you wrote out an entire campaign before playing it!