Les it depends on your age. If you are 95 ,Yes you probably will die before you learn Masons Apron, If you are just a young bloke you probably will die before you learn all of the variations of Masons Apron, but if you only want to learn the two usual parts of Masons Apron It should only take about a day or two. First remember that most session tunes were writen for fiddle and so are in keys reasonably easy for fiddle, ie G & D & A Masons is in A If the tune was writen for melodeon it will probably be in G or D ( But this is a music forum so we shouldn't talk about melodeons here)If you use your mandola in G D A E tuning you will be an octave below fiddle so fiddle keys should be no problem to you. When I learn a new tune I usually get a midi and play it on Cakewalk which also allows you to slow it down without altering the key. Play it slowly at first and then increase the speed till it is faster than usual and you should be able to play along with anyone. Before I had a computer I used to listen to the tune till I knew it well and then played it from memory. Another way is to attend a session and try to join in with others quietly at first. There are mandolin sites on the net which give you tabliture which is very easy to read and gives you positions for fingers.
Bill