It always seemed like a lot of hassle to get on the tour list - and to the best of my knowledge they are only in Japanese - so I never bothered. The gardens and outside are nice and simple to visit. If you try to do the Main Entrance side (where the famous turret view is located) and the East Garden all in one visit, it will involve a fair amount of walking. I got some quite nice photos in the East Garden.

If you are going beyond Tokyo to Kyoto, the Imperial Palace there is much simpler to visit, although the tours pretty much just take you around the outside of the various buildings and through the gardens. At a few points you are permitted to LOOK inside but you can't GO inside. The self-guided tour of Nijo Castle actually does go through the interior.