Choisty session is 4 x 2km as you did the 2 mile session a few weeks ago to test it out think best to do the 2ks but happy for you to do the 750s which should be run around 10k speed but suggest to test control you try and get each one slightly quicker so to do that start slower than 10k pace.
re the debate everyone has different strengths and requirements and agree just because you are in good 10k shape won't help in the Marathon unless you have done marathon type training too but come April on race day when you do the marathon it helps if you are fit and strong enough that you could also do a 10K pb if you were racing that distance that day and I think always good to try and do a 10k race in the build up.
I also agree that when runners get really fit for a marathon they often then go on to run a good 10k once recovered. Paula Radcliffe ran her best times at all distances after she did her first marathon. The stamina gained enabled her to hold speed better
Around that time and up to her marathon World record she was in low 30 10k shape. Because now her 10k potential is probably over 31, I don't think she will run a marathon much inside 2:19 at best.
I recall over the years lots of top quality 10k runners running a great first or second marathon but once they perceived they were more marathoners - they changed their training, they never ran as fast at the marathon again.
Things like distance/pace converters generally are quite accurate between 5k and half-marathon depending on doing balanced training with the right speed, speed endurance and endurance. Marathons are another matter. I did my first marathon in 1976 and generally with a few exceptions, my marathon times were down on what they should have been until the period from 2003 to 2010 when I was averaging around the 2:45 mark when I got the pacing, training and nutrition and mind more in tune with marathoning.
Too many factors influence what you run in the marathon but the converters do give a time that ultimately may be possible in a perfect world.