I'm between the training and the assessment. You're supposed to have done 20 or more 'quality mountain days' before the training course, though some of the people on my course hadn't, and as pedro says you need 40+ for assessment, and that has to be in different areas. Although the log book has a list of suitable upland areas in the UK, Plas y Brenin, where I did my training, were pretty firm that they wanted to see lots of stuff from Snowdonia, the Lakes and Scotland rather than things like the Brecon Beacons, Peak, Dartmoor etc.
Previous experience of wildcamping is useful for the training, and as I mentioned on your other thread, getting your nav up to a good standard before doing the training helps enormously - you're then picking up on other stuff rather than having to concentrate so much on getting the nav right on the course. For assessment you have to have had experience of leading and managing groups on the hills.
One of things I think a lot of people find tough on the ML is security on steep terrain and ropework. I've found doing some climbing has helped a lot with that, getting more confident in selecting anchors for example.
