I started learning Japanese as a hobby in 2004. About a year ago I was diagnosed with moderately-severe ADHD. Since then I've been figuring out how it's affected my life in general and language learning in specific. I'd like to do some writing about ADHD and language learning, but (unfortunately and/or typically) never quite get around to doing it.
You'll certainly want to consider how your ADHD affects your learning style; if you try to follow an approach that doesn't fit or that overloads you, you're going to have a difficult time. Many ADHD people have trouble shifting direction; others have trouble staying focused on one thing at a time; and many of us have both traits. (And some of us talk/write too much

)
Personally, I have difficulty doing one single thing all the time (and completing anything). I need variety, I need practicality and I need fairly quick results. As much as I'd love to devote 2-3 hours per day solely to RTKanji (and nothing else) until I finish, I can't: I'd quit after a week. Ok, after 2 days.
Japanese has been my only hobby (almost) for the last 3 years, to the surprise of my wife who knows that my interests come and go. But learning Japanese suits my brain: I get a variety of experience (audio lessons, doramas, textbooks, conversation, music). Whether I have an hour or 3 minutes or am driving the car I can find some way to get some Japanese, and that learning process feels good. It's the drug that's kept me going.
Variety also gives you lots of repetition from a range of sources, which is good if your brain likes to see/make connections. If I see/hear something 3 different times from 3 different sources, I learn it more quickly than 3 times from the same source. Some days I study my textbook, some days I look at a Japanese magazine, some days I read a grammar dictionary, and some days all of them. If you haven't read it already, check out
http://www.alljapaneseallthetime.com/blog/. It'll really give you some ideas.
When I started Japanese,
Remembering the Kana really suited me. I was playing Shenmue on my Dreamcast and wanted to read the signs in Dobuita. With RTKana I was able to learn the hiragana in 3 hours on a Saturday and immediately use it, then learn the katakana in 2 hours on Sunday. Practical, quick results. Good, good stuff.
I also started with Pimsleur. When I heard the first conversation in the first lesson, I knew I'd never continue with Japanese: I'd never be able to understand that stuff. After repeating words and phrases for 30 minutes (or so), I DID understand it and it was like magic. Practical, quick results, variety of inputs. The repetition really helped drill a core of Japanese into my head, the lessons were short enough to keep my interest but long enough to be relevant (japanesepod101 didn't work for me) Check your local library; maybe you can borrow Pimsleur to use on your iPod.
A good textbook will fit and support your learning style. I've been enjoying
Japanese for Everyone because it has a lot of variety, a lot of content, a lot of exercises, clearly-defined vocabulary lists, and it's challenging. I can work on it for 10 minutes and learn something new; I can always find vocabulary lists to write down and carry with me for memorization.
Finding conversation partners and a local conversation group was a very good thing for me too. Again, variety. Practicality: I could practice my Japanese on real people. And as a telecommuter, I didn't get out of the house much; the conversation group helped me get out and meet new people once in a while.
RTKanji, on the other hand, has been a problem for me. The first few chapters really suited me--learning new things, variety--and I bought the book immediately. But over and over again I've worked at the book for a while, then my brain would get overloaded and I'd stop. A few weeks/months later, start from the beginning, make a little more progress, die. I'm on iteration 5 now. Setting a consistent schedule hasn't helped; it's turning into something I just have to pound away at. I've been tempted to look into other methods--maybe that dreaded combination of form PLUS readings PLUS meanings PLUS compounds would give me the variety I'd need to keep going.
It's important to recognize that your learning style is not the same as anyone else's. As the ancients liked to say, "Know thyself". Be patient with yourself, cut yourself slack when you need to, and don't be afraid to shift direction if the current one isn't working. Sometimes everyone else may seem to be having great success with a particular technique or resource, everyone else may seem to be progressing at light speed, and you're not (and sometimes the converse may appear to be true).
But you're not in a race to learn Japanese. Real life will intrude sometimes, and sometimes your own traits will pull you in other directions. My own experiences have taught me that it's much better to go with the flow and enjoy the journey than to worry about my lack of progress and/or forgetfulness. Stay in touch with your source of motivation; keep reminding yourself why you're learning. Try a variety of resources to see what works for you. Try them all at once if you'd like. Consistency is a good thing: once you find a good path for yourself, find a way to stick with it: a set time every day, a study partner, a deadline, a challenge, or whatever works for you. Keep at it, and you'll get where you want to go.
And that applies to anyone, ADHD or not. I started this with the intention of giving you some insight on ADHD-specific challenges to learning Japanese, but ultimately you just need to do the same as everyone else: figure out what works for you and keep doing it.