I started my PhD just four months back. I've been doing aikido for three years now - I started training just after I finished my Masters degree. I have my 1st kyu certificate framed on my office wall - and I'm more proud of it than I am of my under or post grad degrees. Why? Because its just so damned hard to learn! The nice thing and the fun thing is that once you learm to fall properly (and hence stop hurting yourself all the time), aikido training is about constant learning. Its challenging stuff - frustrating sometimes - but as far as I'm concerned learning is fun, and the more you learn about something, the more fun it gets. If it had been easy, I guess I would have given up training long agi. As it is, I think I'll be doing aikido for a very long time.