I have slowed down somewhat for various reasons, but I'm still progressing with the five languages I set out to learn in August last year. It's not easy. My brain does tire but that's to be expected when you're trying to absorb alien pronunciations, grammar and words. There are differences in difficulty between the languages.
Hebrew has a lot of 'sh' sounds, but the grammar is very similar to English so that makes it a little easier to learn.
Japanese is going ok because I studied it for a few months many many years ago here in Hong Kong. Much of that study still exists in my brain somewhere and that makes learning Japanese now easier.
I tried to learn Spanish while I was in Durango last year. Unfortunately, I didn't have Pimsleur Spanish course material with me at the time so progress was difficult but combined with actually being with Spanish-speaking people did lay down some groundwork for when I began to learn it again with Pimsleur many months later. I was learning from Living Language, which like so many other retail language course institutions simply includes a list of new vocabulary and a 'dead' conversation with each chapter, and puts everything on CD to give it more appeal. The Pimsleur courses are very different. Each lesson introduces new vocabulary but does it within the conversations that occur during the lesson. Sentences and conversations are not 'dead' because they will alter the grammar of the phrases and sentences when it's possible so that you become accustomed to the various possibilities. Word endings and other elements that change the meaning of the words are introduced and explained when necessary and at a slower pace so that you are not overloaded with information, something which normally results in scaring the student away. By design, without realising it, students learn to assemble full sentences from multiple phrase elements simply by following the lesson. The Pimsleur people really put a lot of effort into making their courses work. It's no wonder additional levels can take so long to be released.
Italian is a little difficult because I have absolutely no background in it except for a very few similarities with Spanish; feminine and masculine objects and verbs, etc.; and because Pimsleur doesn't explain everything at once, you sometimes have to accept what you're learning at the time and have faith that an explanation will eventually come. For example, the variations of the 'want' verb take a little while to get used to. As you progress through the lessons though, patterns in the way the language works become evident and you begin to fully understand the language without realising it.
They're all difficult in one way or another but the one that literally hurts my head is Persian. It is completely different to English. Both the pronunciation and the grammar are completely alien to me. Furthermore, they sometimes use phrases and words to say things that we would say completely differently in English. Persian will take a while to get used to. Unfortunately, Pimsleur has only released level 1 of Persian. When I finish this level, I'll go through it again and hopefully soon after, level 2 will be available.
Levels is the one limitation of the Pimsleur method. I tried studying Indonesian with Pimsleur (actually while I was on set in Durango) but the only course available is the 10-lesson "compact" course. You can do very little with just ten lessons. Standard Pimsleur levels contain 30 lessons. Only level one of Persian has been released (perhaps for the armed forces and U.S. corporations working in Iran?) which is fine if you want to learn enough to travel with, but not if you really want a conversation. Fortunately, two levels of Hebrew are available so I should have a decent foundation on which to build once I've completed that course. Italian, Spanish and Japanese are languages in high demand so three or four levels are available for each of these languages.
When contemplating learning additional languages from the Pimsleur library, I now check to see how many levels are available. My minimum requirement is two levels. Other languages that I'm considering include Korean, Russian and Arabic with emphasis on the word "considering".
For those people more dedicated then myself, most of the Pimsleur courses also contain reading exercises. I haven't used them yet. I'm too lazy. Maybe one day...
Oh, and one more thing. My biggest challenge in learning these languages? Rolling r's, used extensively in Spanish and Italian.