How you re-enter the school will depend on the school and instructor. Ask their advice, and be up-front about how much you've forgotten and your current level of fitness. Don't pretend you can walk into a school with a preconceived idea of what belt you'll be wearing and what level you should be training at: that is ultimately your instructor's decision. It's a good idea to think through the options, though, as they may ask you what your thoughts are.
I've been through this experience myself. There are a few things to keep in mind personally as you re-enter your art:
One of the guiding principles of tae kwon do is indomitable spirit. Do not be discouraged when the other students at your level are doing push-ups in sets of 30 and you can hardly manage 10. Do not stop at your comfort level, but push yourself hard to recover your strength and fitness. That said...
You won't be as strong or flexible as when you left, and if you're not careful, injury will slow you down or stop you. There are three things you need to worry about:
- You're probably not as flexible as when you left. Warm up properly, and don't be tempted to rush your flexibility training. Especially when doing things like high-heel/axe kicks, be aware that your brain remembers kicking higher than you can now, so build up to it gradually.
- You're also probably not as strong as you were. This can result in a different kind of injury. Be careful with your kick pull-backs, especially. Work on strengthening your hamstrings. Trying to throw a kick with more force than you can handle will result in injury.
- Particularly if you haven't been doing other exercise, your general level of fitness may not be as good as when you left. Train hard, but don't over-do it and don't let yourself get run down. You may need to improve your diet (more healthy food, less junk).
Also keep in mind that a tae kwon do class is (usually) not a complete personal training regime. Your instructor can probably offer advice, but schools often expect students to do their own strengthening, stretching, and cardio work outside class. You may have extra catch-up work to do!
As always, if you're older, have heart or other medical conditions, or have a history of recurring injuries, it's worth talking to your doctor as well - not because he might say no, but because he might have suggestions specific to your medical history for getting back into things.